Fun concept. Small little creature turns into rampaging monster.
The problems with gnar, is that he is super oppressive. In lane, he is an annoying range abuser like quinn. He is almost guaranteed a free winning lane for no effort. He also has insane chase, which is next to impossible to escape from once he starts autoing you. He is only hurt when he turns into mega gnar, because he becomes weaker, but dont let that fool you, all he has to do now is start chaining any of his abilities because they all have cc now. He also wins almost every one vs one, so he can splitpush like a god.
Gnar is good at:
It is hard to think of a champion who is so good at all jobs. There's a reason he is always meta in pro play.
I do not think nerfing gnar is the way to go. If he isn't god tier, he will be garbage tier. I think the only way to salvage this great concept is to rework him, possibly making his mini form melee so his lane is less oppressive. Until then nerfing him so the overloaded blight in top lane is less oppressive would also be good.