Hello,
Currently I am Gnar main (mastery 7) and I would like to share this build for my little cutie.

Gnar is a mobile champion with a lot of poking power in the mini form and very tanky, CC (up to 3 second stun and perma slow) and burst oriented champion in the mega form.

All his skill set makes him perfect toplaner, being as big lane bully in mini form, as strong monster in mega form. He is quite annoying for enemy in any situation. But has some problems. This build is made to fix them, as well as to increase all powers, which he currently has.

As the season 8 came, runes reforged wee implemented. Now they actually do something to the game. But what to choose?

My idea was inspired by official Riot video: Play Video
Although, I decided to change it a little bit, to emphasize on champion weaknesses even more, rather than just addition to something good he already equipped with.
This build bases on bringing a lot of free gold to your pocked, giving you opportunity to go ahead of your opponents. ms/ad runes, are giving him all-rounded utility throughout the game.And last, but not least is Glacial augment gives him free frozen mallet, what was must have on Gnar. Now you can go another offensive item, while still being very slippery and powerful.

In other words, your laning faze becomes good starting from 2 level, and, despite all other build, this will make Gnar viable even during late game, cos of 7 items build and gathering storm, giving a lot of AD, if the game is not over yet.

Gnar q () is long ranged skillshot, which slows enemy. In mini form it deals mediocre dmg, applies stack of w and is an aa reset ability. You need to catch it on its way back to get some cd refunded. (tipp - next your right click on the ground will be place where q returns. do not forget that!)
In mega form, it deals high dmg, and refunds cd, if you pick up the boulder. (tipp - use it close combat to get some nice burst, slow target, and immediately get cdr.)

Gnar's w () in mini form is basically vayne's passive. If you hit enemy 3 times, you get nice bonus dmg on then, calculate from max hp. But also it provides decaying ms buff for you. (tip - use aa, q, aa combo for a good trade.)
In mega form, it is AOE stun in a line for 1.25 sec with nice dmg.

Gnar's e () is position jump, which dmg based on your hp. It slows target and you jump second time in mini form. In mega zone is bigger, but you do not jump.(tip - wait when you have 100 rage, use e and you will get effects of both mini and mega e.)

Gnar's r () is passive in mini form, giving you higher cd reduction on q and higher ms baff on w. In mega form, it is AOE displacement around 1 meter of you. All caught enemies will be trown after 1 sec in specific direction, dealing dmg and slowing them. If you hit them with wall, they will be stunned for up to 1.75 sec. (tip - use it behind yourself, and all enemies (even these, which were in front of you, will be displaced on a quite a long range to the opposite direction.)

Early levels: e, aa, q, aa, w, aa, aa, q

Mid game (without trinity force): e, r, aa, w, aa, q, aa

Late game (with trinity force): e, r, aa, q, aa, w, aa, aa, q, aa

Be pure gentleman -- CC them to death!

Pros:
- High mobility in mini form
- Good dmg/poke
- Durable in mega form
- High CC
- Good lane sustain (hp regen + passive boost, porting of which is left after the transformation)
- Manaless

Minuses:
- Falls off a little bit late game (30+ minutes)
- Low HP in mini form
- Low catch up potential in mega form
- Low CC in mini form
- Needs items to do well

Build solutions:
- Runes only make him stronger late game
- Quite durable in mini with items
- With approach velocity + Glacial + ms runes be hard to escape
- Free frozen mallet in mini form from the begging of the game
- free effective gold from runes

To conclude, Gnar is very cute champion, as by his appearance, as by his gameplay. He requires a lot of practice to truly master him, but it definitely worth trying.

Good luck on the Rift