Hello, My name is Forresto, and I'm currently a gragas tank main. I started this after I got trolled by a gragas top who was absolutely unkillable. Curious, I tried it myself, and after some experimentation found my own preferred build.

The goal of this guide is not just to offer a set of items and runes, but to give you an in-depth understanding of how to play Gragas. More importantly, I hope to give you understanding on why to do things. After all, knowing why to do things is more important than understanding how to do them. As such, I fully acknowledge that it's pretty wordy. If you want to skip to the technical ins and outs of Gragas, I've placed them at the bottom.

I'm going to be honest, I love playing sustain based champs. Who doesn't right? But most sustain based champs have a fatal flaw. They have to interact with the enemy in order to sustain. This is all well and good if you're already breaking even. But if you're losing, then things like lifesteal aren't likely to make much difference because in order to get the healing you desperately need, you have to expose yourself to more risk.

And thus we have Gragas. On a relatively low cooldown, he can continuously pump his hp back up with his passive. As long as you don't die you can recover from any trade, no matter how good or bad. Not only that, but he is a nightmare to fight against. With a single combo, he can chunk enemies for anywhere between 1/3-1/2 of their HP despite building full tank. What's more, my preferred combo gives the enemy almost no room for counterplay. It's hard to dodge, and by the time they come out of the cc, you've already walked away. He just has so much CC and mobility baked into his kit. Even if the enemy does outplay you, you can just walk away, chug your W a couple times, and most-all of the bad trade is wiped away.

Basically, laning against tank Gragas is an impossible no-win scenario for most enemies. If you want to be an obnoxious prick to your enemies, Gragas is the champ for you.

So, the first thing you're likely to think about when building your champ is runes. Here, we take the green resolve tree primary, and the blue sorcery tree secondary.

Resolve:
-Grasp of the undying. It adds a ton to your skirmishing and fighting power. The damage is great, the healing is ok, and the stacking hp is a nice little cherry on top. Aftershock is also viable, but offers a lot less flexibility, on a much longer cd.
-Demolish. Mostly because it's the only really useful rune in this row. You're a solo lane so there's no point to font of life, and you have no native shields, so shield bash isn't going to do anything for you until ~20 minutes when you finish fimbulwinter. Without demolish, you present 0 threat to towers, but with demolish, you can at least take a plate if you get the chance. Take shield bash if you really don't care about the extra gold from plates.
-Conditioning. Any of the runes in this row could be good. But, here's the thing. Bone plating has a 45 sec cooldown, and is very easy to purposefully poke out, so the value you get from it over the course of lane is miniscule. You should be sustaining your way through lane anyway, so it's not suited for gragas. Second wind is actually a very good rune for him though. The reason I prefer conditioning over second wind is that he has so much potential healing just via his kit that he simply doesn't need more, making the mid-late game bonus resistances more useful.
-Overgrowth. But Forresto, if we're supposed to sustain through lane, why not revitalize? Simple. His healing is % max HP healing. You certainly get more overall healing from revitalize, but overgrowth will increase your hp, and thus give you some extra healing while being a bit tankier. It's simply more efficient. Honorable mention to the unflinching rune because who doesn't love tenacity?

Sorcery:
-Manaflow Band. Gragas is mana hungry until you're pretty far into charging tear. Manaflow band helps with that, and once you stack it fully, the mana sustain it offers will essentially solve all mana issues for the rest of the game. Also, it increases your max mana, which gives you more HP and shield power from fimbulwinter.
-Gathering Storm. This rune slot is actually a tossup between gathering storm or transcendence. I can really see an argument for either one, but I feel like tank gragas benefits from having a bit of extra ap as he scales into late game, and people build more defensive items. It's up to you which you prefer.

Minor Runes:
-Adaptive Force
-Adaptive Force
-Armor
The only wrong choice here is attack speed. Any other minor rune is fair game depending on your preference. I personally like this setup because it gives his trades a little more oomph in the early game, but you can mix and match according to your preferences.

-Flash
80-90% of champs take flash, and gragas is one of them. His main combat tool is a dash , and flash has a long list of things it offers. Everyone except shaco needs at least one mobility summoner, and you have no special synergy with the movespeed from ghost, so take flash.
-Ignite
I prefer ignite. It makes up for the lack of AP in this build, and especially adds a lot of kill potential against champs with decent in-fight healing; people who are often fairly good at dealing with your skirmish damage. Also helps with finishing off those people who stick around under tower when they're way too low and should leave.
-Teleport
It recently got nerfed. Epically nerfed. More importantly though, you likely won't be leaving lane that often. The only time you should be out of lane is if you run out of mana or die. You're exceptionally beefy, have good anti-gank escape tools, crazy sustain, and have enough firepower that you can frequently catch enemies off guard for a free kill during ganks, so as long as you dont get cocky, you shouldn't die that much during lane. Later in the game, Teleport is mostly for either defending towers against unexpected pushes, or joining your team. Since you have very little kill pressure against towers, you shouldn't be splitting, and thus should always be with your team anyway, so the only thing teleport really offers is the ability to defend against a surprise push. It's up to you which you take, but I personally prefer ignite.

The tooltips at the top of the page will tell you what the abilities do, so I'm going to use this space to explain why I chose the order in which I allocate ability points.
-Q
Since I don't have much AP, this ability is really only useful for the slow. Not only that, it costs as much mana as both your other basic abilities combined. As such, I put 1 point into it at either level 3 or 4 depending on my mood, and leave leveling it for last.
-W
Your primary means of activating your passive, and an important damage tool. E is more valuable though, so I put a point into this at level 2, and max this second.
-E
Your most important basic ability. It's a strong mobility tool, strong cc, and your best damage all wrapped up into 1 button. Max this first because not only does the damage go up, but the cooldown decreases drastically as you level it. keep in mind that the cooldown is effectively 3 seconds less than the listed number in most situations, so the decrease in cooldown time is more significant than it seems.
-R
One of the best abilities in the game when landed properly, and one of the hardest to land properly. The resulting CC is massive, single-handedly changing the topography of a teamfight, or turning a densely packed formation into a free kill. Just remember that the difference between sending the enemy flying into certain doom, and flying towards the safety of their tower is approximately 1.5 nano-pixels

Starting Items:
You should start with a tear of the goddess, and 2 potions. Why not Dorans ring for more damage, or dorans shield for more sustain? Because tear of the goddess does both things better. Sure, each trade will do a bit more damage with a dorans ring, but you're going to run out of mana a lot faster, even with the moderate mana regen bonus. Tank gragas lane phase is largely a war of attrition, so less trades = less damage overall. On the other hand, Doran's shield seems like it would give more healing, but gragas passive can give him ridiculous out of combat healing potential. Since it costs mana to use his passive, you're going to get more overall healing just by virtue of having more mana, rather than relying on the bonus healing from a dorans shield. Keep in mind that the entire healing cycle of a dorans shield at max value (critically low hp) is actually worth the same as your level 1 passive, so you get a lot more value from having more mana to keep spamming your passive than you do from being out of mana with a dorans shield. Also, fimbulwinter is absolutely core, so the earlier you start charging this the better.

Core Items:
-Frostfire Gauntlet.
Your mythic. For those who don't know items by name, this is the tank mythic that causes an aoe frost terrain effect that slows enemies when you auto. Once you're a few items and a number of levels in, your cooldowns will be so low that combined with this item, you can essentially keep an enemy permanently cc'd. (q slow, e knockup, frostfire slow field on a constant rotation) This item gives a lot of value. You already use autos in your combos, its good for peeling for your allies, its good for sticking to enemies, it adds extra damage, the tank stats are good, Gragas gets massive if you reach full build. (Aside from being hilarious to look at, this actually gives some extra width to his dash) Great synergy all around.
-Fimbulwinter
The tank item evolution of tear of the goddess. You know how in mid-late game, your passive is healing for 1-300 damage every 8 seconds? Well now you get ~200 hundred more effective HP via shield, once every 8 seconds, plus over 600 raw hp (which feeds back into your healing. This item does massive things for your ability to tank your way through teamfights. Remember, the less damage you take, the more that 8% max hp from your passive matters in terms of long-term sustain in a fight. (also, the massive amount of raw hp stat will feed into the damage from your grasp rune, and frostfire)
-Demonic Embrace
So, you've spent the last twenty minutes mostly stacking HP items, what now? Demonic embrace is what. Of course, going absolute full tank is great, but by this point you could really use a bit more oomph in your build. This item is perfect for that. You get more HP from it, you get AP that scales with your bonus HP, and the burn effect is great. Just a great item for evening out Gragas's performance in teamfights. Once you finish this item, you should be able to hit squishy carry champs for half their max HP in a single combo, while being a massive body-blocking, ****-blocking tank champ. It's an absolutely great feeling, and I highly recommend trying this out.

Post-Core Items:
So, you've finished your core items, and no one has surrendered in the face of your girth. What next? These 4 items are the best general catch-all choices for specific common scenarios.

-Thornmail
Do you need armor, and they have 1+ champions with lots of healing? (or you just want to build against their adc) Thornmail is great for offering more heal-cut at times when your ignite is down, and adding some extra damage with no extra effort on your part.

-Dead Man's Plate
Do you want armor, but you're not really that worried about lifesteal or their adc? Or just want another armor item to add to thornmail? This is a less common scenario, but generally it means their adc has been getting wrecked all game, while their comp is mostly AD. Why choose dead man's here? Well, more mobility is always good. There is no champ in the game who doesn't benefit from extra movement speed. On top of that though, you essentially get an extra sheen effect. Remember how your autos are doing your w, and grasp, and frostfire damage on top of base ad? That's right, we're adding another 100+ on-hit damage to your initial burst while building full tank. Also the on-hit slow is slightly stronger than frost-fires slow. So, good synergy with gragas, the tank stats are a bit low but still acceptable, not a bad item to cap your build with.

-Spirit Visage
Is the enemy lux absurdly fed? Did you ever wonder what it would be like if your passive and fimbulwinter were even more ridiculous? Then this is the item for you. It even gives a bit of ability haste to keep the cc chain going. This adds an extra 2% max hp per passive proc, around 50-60 shield to fimbulwinter, .5% max hp to grasp healing, and a marginal amount of the health regen stat. These might not sound like much by themselves, but you can reasonably proc passive and fimbulwinter twice in an extended fight, and more when fleeing during a long chase, so the extra healing/shielding can really add up. (not to mention the further added value if you have a heal/shield bot like janna or soraka on your team) This is probably the best all-around MR item for gragas since your tankiness is so intrinsically linked to your healing/shielding.

-Abyssal Mask
If you want an MR item that offers more damage instead, abyssal mask is a great go-to. It passively reduces enemy MR based on your max HP up to a max of 20. This is great for boosting your damage, but it's also useful for your magic-based teamates. A good pick if both teams have a lot of AP champs, or just want to boost your personal damage against a team with several casters, but I love playing the sustain tank role, so I usually go visage if I need MR. Naturally, the choice is yours.

Boots:
You're a tank, so you basically have two solid choices. Tailor it to the whole enemy team, and which champs are fed, or potential threats. Don't just react to which damage type your lane opponent uses. (unless you're absolutely struggling, and need the help to survive lane)

-Mercury's Treads
There are two reasons to choose these boots. For the tenacity, and for the MR. I would honestly say that if the enemy team has a lot of cc, it might be worth taking these boots even if the enemy team doesn't have a lot of AP. Remember that surviving an extra half second in a fight because you went plate boots against AD isn't going to matter much if you spend the entire fight cc'd.

-Plated Steelcaps
There are two primary components to this. Armor, and the passive effect that reduces damage from auto-attacks. Naturally, you should prioritize these boots against heavy AD comps, and auto-attacking hyper-carries like master yi or vayne. (there are going to be times where other lanes feed these champs, and there's not much you can do about that besides build tankier, and hope for the best)

Basic Lane Playstyle:
Tank Gragas top is a fairly unique experience. Why? because gragas's design is an absolute monstrosity to play against. Did the enemy dash at you? press e to interrupt their dash, slap them with grasp, and walk away. Is the enemy exposed while farming? slam into them with e, slap them with grasp, and walk away. Not worried about being bursted? press w, then slam into them with e, then slap them with your empowered auto. stick around for another grasp proc if you think you can get away with it. There's very little counterplay available to the melee opponents that make up most of the top lane roster besides hide behind minions, and melee champs that are hiding behind minions probably aren't farming that well, which becomes another advantage for you who has nearly infinite mana. (and thus very good waveclear) Your mindset should be a war of attrition rather than looking for a single decisive fights.

But wait, there's more. There's a reason I called this a war of attrition. Not only does Gragas have one of the most powerful self-sustain mechanisms in the game during lane phase, but it's outrageously, absurdly cheap. That's right. For the low low cost of 40 mana per activation, our passive will potentially heal us at almost the same rate as garen's healing passive. But wait, there's even more. Because unlike Garen, nobody can disable our passive. Once every 8 seconds, we are guaranteed to get a healing bump equivalent to ~5 seconds worth of garen's passive. This may not sound like much, but consider that aside from pressing the button to activate an ability, this healing requires absolutely no interaction with anything. You don't have to put yourself in danger trying to lifesteal from minions. You don't have to perfectly land a darius Q. You don't have to timidly sit really far back as garen trying to avoid getting poked. You can be exactly as aggressive or timid as you want, without sacrificing anything.

What does that mean in practice? It means that as long as you don't die or run oom, almost any trade you make is worth it. Did you get chunked for 400 damage, but you only did 200? that's ok. pop your passive 2-3 times, and it's as if you won the trade. More likely though, what happens if you're experienced enough with gragas is this: you either consistently trade evenly, then completely nullify most of the damage you received with passive, or you frequently find the opportunity to one-sidedly harrass your opponent. If you charge grasp, use w, then land an e/auto combo, all you have to do is walk away. Just that will chunk them for a monstrous amount of damage while giving them pretty much no opportunity for counterplay. Then, when they're low enough, it's generally not hard to surprise them with a full extended fight. e-w/grasp/auto-q-ignite, more autos with more grasp procs, ult if you think you need it for either cc or extra damage. Even without building ap, if you're smart about how you fight, you have more than enough base damage to give most champs a run for their money.

Ganks:
Red alert. A second champ has invaded your lane. What do you do? You have a few choices, and they all revolve around your e cooldown.
-Option 1: dash away.
This is the safer option if the enemies are either spread out, or far away. Just do your best to run back to your tower.
-Option 2: dash towards.
So, the enemy either already reached you, or has a strong engage tool like a dash. You don't think just pressing e will be enough to get away. You're not necessarily dead. If the enemy is already on top of you, instead of dashing away, you can dash into them. Remember that your E isn't only a mobility tool, it's also your primary damage source, and a knockup. By hitting them with cc after they just used their engage tools to get onto you, then running away, you give yourself the opportunity to make space again, which will often spoil ganks. Alternatively, you can actively interrupt most dashes by slamming into them, potentially further spoiling a gank. Just be aware that there are two people in the lane.

-The Nuclear Option
Your ult is one of the biggest ****-block CC tools in the game. If you really think you have no other choice, use your ult. Better to lose some kill pressure for a minute and a half than to give kills to the enemy. easily 60% of ganks can be ended just by pressing R and running away to chug yourself back to full HP.

Early game is over, you've either killed their tower, or lost your own, and its time to roam the map with your team. What do you do though? what's your role? You have one of two choices, and at least one thing that sounds good, but which you shouldn't do.

-Engage Tank
You are a true man. You make your enemies wail in fear, and take joy in their cries. In other words you like to chug a barrel, then slam it into someone's face. You have two modes as an engage tank. One mode is "I have my ult." The other mode is "I don't have my ult." If you have your ult, then you are the god of teamfights. Your role is to look for a moment where someone is just in the right position to knock them into your team by themselves. This works well with teams that have lots of burst or cc, and can capitalize on this. Avoid doing this to champions like Ekko who have uniquely easy escape tools since you're usually just wasting your ult in those cases. If you don't have ult, you're not useless. You don't have the same god-like pick potential, but if you find the right situation, you can force the enemy team into disadvantageous fights. Basically it goes like this. The enemy adc is out of position. Maybe they took the wrong left turn, or maybe they're chasing another team member. So you slammed 400 pounds of fat, greasy, drunk alpha male into them, and bashed their head in with a barrel of grog. Now their team is forced to turn on you. After all, they can't let the adc die. So even though you might suffer in the short term, your team has just been put into an advantageous position. You've just bursted half of the enemy adc's HP, knocked them up, slowed them with frostfire, then forced out several potentially long cd's, possibly even ults, while hopefully not taking too much damage. Your team should now be able to (hopefully) capitalize on this opportunity, since you have 4 teammates, and there should be 2-3 enemies focused on you, (the tank) leaving your team of 4 to hopefully quickly kill the 2-3 that aren't piled onto you.

-The Peel Tank
Ok, maybe you don't want to be so aggressive. Maybe your adc is 20-0, and your time is better spent helping the support keep them alive. Maybe you just really don't want to get nuked by darius for the 15th time. Whatever your reason, this is the less agressive, more team-focused, objective-oriented playstyle. Basically, you are the king ****block of ****block mountain. Your E knockup is on a short cooldown, your Q and frostfire are on short cooldowns, you have ult as a nuclear option. You are very much capable of keeping any champion you choose in a nightmare of permanent cc, then knocking them away with ult after they burn all their mobility skills. If you're not confident in your skills as gragas, your skill at landing ults, or your ability to read the proper time to engage, I highly recommend this playstyle until you feel more comfortable.

-What not to do.
"I have amazing waveclear and dueling potential." says the eager gragas player. "I should split push. I want to farm a bit more." Well, ok, maybe you can go farm a wave or two during downtime, but you absolutely, definitively, do not want to split push. Here's the thing. You are very good at getting to the tower. You are very good at running or fighting when people respond to your presence at the tower. What you can't do is any damage to said tower. (aside from the demolish rune, but several minutes in a side lane isn't worth doing 1/5 of the tower's HP) Not only that, but if you followed my build, you probably have ignite, meaning you're not able to be with your team at other objectives, or fights. Also, gragas is so amazingly useful for both utility and damage in fights that unless you pulled their 20-0 adc into the side lane and away from a teamfight, literally no one you can pull is more valuable than the value of you being present at the teamfight. (seriously, your ult alone provides more teamfight value than most entire champs) So just don't split. Stay with your team, pick a niche between peeling/engaging, and fulfill that role.

-General
*If you've shoved someone to their tower, you can burst-poke them at very little risk. (this is only if theyre in the front half of their tower. dont try to dive the back half)with your usual e-auto combo, you should only take 1 tower shot by doing this, which is pretty much the same as the healing you get from 1 passive proc, making the trade essentially free for you.
*If someone is hiding under tower, but you know you can kill them, you can bring them into range for your combo by using your ult.
*To expand on that, if someone is flying at you due to the effects of a cc, including your ult, you can wombo-combo them with your E. What's better than the decent damage and knockback from your ult? Doubling the damage and cc by using your E. Also, it's hilarious to watch.
*You can flash partway into your E, and the dash will continue for its remaining distance in the same direction. If someone tries to dash away, you can flash onto them for an unfair, undodgeable E. Obviously this costs you your flash, so save it for guaranteed kills.
*avoid using your Q as much as possible. Even with your high mana bar, your Q will quickly deplete your mana if you constantly use it. Since we're not putting any points into Q anyway, it only does around 60ish damage, so unless you need the slow to chase/escape, it's just a waste of mana for regular trades.
*being chased isn't necessarily a threat, it can also be an opportunity. Since your combo inherently offers very little opportunity to counterplay, anyone who solo chases you is likely making a huge mistake. The scenario goes something like this. Let's say nocturne ults onto you for some reason. Your natural instinct might be to flee in fear (pun intended) but you would be wrong. Don't flee in fear, make a tactical retreat. Every time your E is up, you should w, bump into the enemy, auto, and walk away. If they keep chasing you? flee in circles until your e comes back up again. bump and run again. W-auto and run. Q and run if you need more breathing room. just keep chaining your abilties backwards at the person chasing you while never sticking around to let them hit you. Keep their naive dream of killing you alive by staying just close enough that they think they have a chance while you burn them down. Naturally, you will take some return fire, but Gragas is a big fat sustain tank. It's highly likely that if you do take damage, you can just heal it back with passive. I can't tell you how many times someone has chased me until I burn them down to half HP, then died as I suddenly full-combo them with my ult and/or ignite. Remember, being chased is an opportunity. Disclaimer: don't try to skirmish like this against two+ people unless one of them is extremely low HP. The success of this is based on your ability to chain CC someone while keeping them at the exact right distance to maintain your safety while threatening theirs. If there are multiple enemies in the equation, it becomes significantly harder to accomplish this.

-Jax
Believe it or not, jax is one of the easier lane opponents. He simply has no way to fight you as long as you play smart. Don't be overly aggressive against jax. Instead, hold on to your E. If he jumps onto you, interrupt his dash with your own, then walk away. If he activates his E and walks up to you, dash into him and walk away. The knockup is almost the same duration as his E. (1.5 seconds vs 2 seconds) Either way, he'll be left unable to follow up, while you get away having done some free damage. So, knowing you can do this, you want to be standing at the front of the minion wave. Either he continuously takes bad trades, or he ends up unable to farm. Either one is a win for you.

-Teemo
Just go balls to the wall aggressive. Most teemo players are focused on poking you, but his movespeed bonus is usually not enough to dodge your E with any kind of reliability as long as you aren't fishing from max range. More importantly he has no inherent sustain. By aggressively taking smart trades, you should chunk him down in no time while shrugging off his damage with your sustain.

-Darius
One of the few people equipped to really win lane against you. He hits like a truck, his bleed counters your healing, he has decent sustain, his execute ult is a good counter to sustain-tanks, he's just really a pain. Do your best to play passive, and farm the best you can. Most likely you'll be stuck under tower using abilities to get whatever minions you can, and that's fine. as long as you're getting gold without dying, you're good. If darius pulls you in, I recommend counter-trading into him into him rather than just dashing away. (e-auto-walk away) This way, instead of one-sidedly taking damage, you trade semi-evenly. Your sustain is still better than his overall, (12% missing hp vs 8% max hp, plus you don't have to worry about hitting an ability like he does) so with enough of these trades, as long as you dont let yourself get into protracted fights where he stacks his passive, you might even be able to pull off a surprise kill with your ult and ignite.

-Garen
Garen is annoying, but not unbeatable. Winning lane basically comes down to understanding and abusing his engagement pattern. By that I mean that if he wants to be aggressive, garen has to start by running at you with his Q. Guess what you want to do then? That's right, slam into him with E to interrupt his engage. (if you have enough tactical awareness, you should be able to activate W first for the extra on-hit) While I've been referring to your E as a knockup this whole time, the truth is it has a very mild amount of knockback effect as well. This knockback is just enough to put garen's spin out of range to damage you, so in the end, the only damage you take should be from his Q. By trading this way, as long as you don't have a super long fight with him, you should eventually force him to retreat, and rely on passive heal. At that point, repeatedly use Q barrels to turn off his passive. Also, it's important to always be aware of how close you are to him. if you don't give yourself enough space, you wont have time to react. if he lands his silence before you press your E, you're going to get chunked for a lot of damage. if you were already missing enough hp, it might even be enough to finish you with ignite/ult.

-Kayle
Go ham. Other than a minor healing skill, she has very little ability to deal with you pre-6. More importantly, you absolutely want to delay her farm as much as possible. Just be careful. The easiest way to lose lane against Kayle is to get so focused on harrassing her that you expose yourself to the enemy jungler. Try to practice proper wave control so that your minion wave doesn't overextend. This is absolutely the most important lane for understanding proper wave management.
If you don't understand what I'm talking about, try this guide: https://mobalytics.gg/blog/wave-management/