I've played him a few times to get a feel for him... he seems a bit ridiculous but he has clear cut weaknesses.
For one, while against Vayne you can sit out the W proc and avoid losing trades that way, the boomerang + stacks on multiple targets doesn't allow that against Gnar. Gnar will pretty much always get a best case scenario in terms of the damage he deals to you in short, medium, and long trades. Best thing is, after the trade you can judge if you want to press further or let it be with the Hyper! movespeed.
It's noticable that people's inexperience makes playing against Gnar harder. The transformation is so hella telegraphed that you will never get an optimal Mega Gnar combo with Hop Crunch against good opponents. I would recommend abusing bushes because it seems to make it harder to keep track of his rage.
Later on you will probably not engage much outside of flanks (which are strong if jungle creeps are around to manage rage) and by joining the midline, however, you still have a permaslow in the backline. His teamfighting seems to be difficult. Keeping track of W stacks, catching Q, positioning (400-500 range...), AND your rage is hectic, but not optimizing it is not the worst. That said, I can imagine optimizing it will make you even stronger in teamfights. I think you're a backliner generally with /some/ flexibility, mainly because your ranged form can shred tanks and melee form is a cc bot.
Weakness? I played against an Akali and she pretty much one shot me in mini Gnar, and sat out a lot of mega Gnar in shroud. I can imagine most assassins/Irelia (whom is kinda also an Assassin so whatever) destroy him (with some exceptions ofc), because mini Gnar is probably squishier than Sona. Yes you have a jump, but CD is still high and if they get some burst off before that, you're still left at like 30% HP without a lot of innate sustain. (you can count mega Gnar but that still opens you up.
Build I used:
Brut > Ionian > Bork > Randuins > Triforce/Youmuus/Visage (in whatever order) > Flex spot.