Hey Guys,
I am a main Gnar Top, currently Silver 4 and I carried myself as Gnar, from Bronze 4 , besides being my first season. Gnar is a very fun to play champion, but quite hard, because there are some things at Gnar that you just have to do, so that you can play well with him.

As Runes, I take the Standard AD marks as I like, and you probably have some too, here is my suggestion, if it helps you ,great, if it doesnt, just skip this part.
MARKS
As marks, I like to take 9 AD marks, because they help you early game and basically apply to every AD champion you want to play.

SEALS

For Seals, I take 9 Armor Seals, that help me early game, because of Gnar's terrible early game, besides, they help you with other champions.

GLYPHS
I like to take 9 Magic resist glyphs , for the reasons told below.

QUITESSENCES
For the Quints, it changes a bit, but I like to take 2 AD quints and one AS speed.

As masteries, I like to take 12 ferocity and 18 resolve, finishing in Grasp of the Undying.
Some people prefer to take 18 in ferocity and take fervor of battle, but I dont like it, because I think grasp gives you some survavibility.

Early Game
This is where you have to be great at. Play careful and dont lose much HP. Last hit only, so that you can enrage into mega Gnar at lvl 3 and perform your e-w-q combo and take some good hp from the enemy Champion. After your first back items, you should already, or do some damage, or are already tanky, so you can take some risks, and still survive. However, you still have to be careful.
Mid Game
If you get there with some CS and your enemy laner isnt fed, this is your time to shine. You probably have finished the first item ( BC or Frozen Mallet) and you do damage and are tanky. After first Item , just go full tank and build against the enemy team. (If they have mid AP fed, my suggestion is spirit visage. If they do more AD damage , do Deadman's plate, or if they have a lot of critics build randuin's Omen.
Late Game
If you have a nice build, you are unkillable, my favourite two last items are Guardian Angel, because of the armor, magic resist and obviously, the passive, and Warmog's because of the passive and it makes you extra tanky.
In late/mid game, there is something that is key in playing Gnar. You basically outduel everyone if u kite, and your ultimate is gamechanging. Try to always be there for teamfights. Late game, farm isn't very important, as Gnar.

PROS +
+ You are a ranged Top laner.
+ You can kite and poke
+ Fun to play and to learn
+ Tanky late game
+ A god mid game, and can basically make any top laner useless, if you play well

CONS -
- One of the worst early gamers
- Not very easy to learn
- Hard to master and to carry
- Only has 2 abilities in mini form
- Not tanky early game

Here, i will leave a few links of some of the best gnar plays. They are all basically the same, because the player just E's , does a good flash or a good hop because he is at the blink of transforming, and stuns the right people. THAT is gnar in every team fight, if you ult well, you did good, if you dont, you didnt. It's as simple as that :D
https://www.youtube.com/watch?v=2-YMc6BR3HY
https://www.youtube.com/watch?v=Oy_PCeymAeY

In this Chapter, I will explain Gnar's biggest Counters, and explain how to play against them, and why are they so hard for Gnar to play against.
Gnar's counters are champions that can do a lot of damage in a short time, and have a good gapcloser.

---

The first counter I am going to talk about is Riven
Riven is probably one of the hardest toplaners to play against. She has a lot of burst and CC, and can easily snowball. However, when you get to level 3, your game becomes easier. If you just E away whenever she tries to Q you, you can deny her the chance to kill her, and punish her if she uses all her gapclosers when she tries to come and kill you, by hitting your Q and stacking 3 AA's on her.
If you are not careful, she can make you have a hard game.

---

The second champion i'm talking about is Yasuo
Yasuo is a very hard champion to play against. He does a lot of damage, but unlike any other counter of yours, his dashing makes your task to attack him when he is coming back very hard. You can outduel him easily, but you have to hit your Q. If he takes your Q with his W(windwall), he can easily kill you, since you are useless without it. Before level 3, you can do some pretty damage to him, but after that, you need your Q to farm, and you cant afford only having it every 20 seconds, so don't poke him, unless you are sure his windwall isn't up.

---

The Third Counter I am talking about, is Pantheon.
He is, like the other three, an assassin. However, you have a easier job with him than with the other ones. He can do a lot of damage, but the minions can even the score early game. Besides, if you proc his passive, you can actually win the trade. If his passive is up, try to AA before he engages so that he doesn't block your AA. If you aren't careful, you are tottally loosing the lane, as he can become very annoying. However, and unlike Riven and Yasuo, you can kite him easily, but to do so, he has to perform his combo on you, and that can make you take too much damage. Late game, you just outscale him, and once you get some armor, it's easy to duel him, since there will be some time in the game you can even let him AA as mini gnar and still win the fight. However, that time will be mid/late game and you have to perform very well early so that you have the advantage, whether in cs, level or Kills.