Peaked at #2 early Season 10 so I have some experience on the champion :)
https://www.leagueofgraphs.com/summoner/euw/ikewolf

The hardest part about gnar is to manage Mega. So here is some tips on how to play around it :

Laning phase

This is probably the most straight forward. Level 1 you are generally very weak in most matchups. Just farm and try to keep your rage to transform as you get level 3. At this point, you can probably trade. Be careful to not use your jump forward if you don't have knowledge of the enemy jungler.

From level 3 to 6, try to harass the enemy as he comes for creeps. Usually the combo is to go AA->Q->AA as he tries to farm. Do not do this if the enemy can engage on your mini form cause you're most likely to lose. Do it only if you can leap back safely.

Teleporting

Alright so teleporting botlane is quite OP nowdays, but very hard with gnar due to his rage. There are a few conditions to meet to be able to teleport :

- Your lane is pushed under the enemy tower
- You are above 50% rage
- There is a ward far enough that the enemy won't be able to just run away

Usually, you can teleport with no rage if the fight is very close cause you will be able to clean up anyway.

TeamFights

This is the most important. When you know a team fight is about to break, you really need to be at about 80% rage. Usually what I do is that I hit jungle camps until I reach 80% rage, wait to see if there is an engage, and go back hitting monsters or minions if I go below 50%. Your team needs to understand that you arnt a front lane at all in mini.

If you see an opportunity to engage yourself, start hitting whatever is in front of you and E into R as you transform into mega.

If you are pushing a lane and about to go mega when your team is preparing drake/nashor or whatever, it's better to give up a few creeps and not transform into mega for nothing. The gold value you get from a good mega teamfight is way higher than 100G

Gnar's ultimate has a very low CD once he reaches lvl 2 (lvl 11 globally). So dont be afraid to use it to overkill, push a wave as you transform back, or even if you're not sure to hit an enemy.

You just need to be careful about using it between lvl 6 and 10, as you won't necessarly get it back when you get Mega again.

Be careful about Sylas wherever he is. Gnar is one if not the best ultimate he can steal, since he can use it whenever he wants and you need to wait the mega form. It makes teamfights for the enemy team much more frightening