I'm no Gnar expert, and this is just my observations from playing as him every once in a while:
In mini form, Gnar's only strength is stopping engages. If you can get him to blow his E, ganking him or even just engaging on him solo is much much easier. His Q poke is fairly weak, because not only does it not do much damage, its also fairly easy to dodge, and it has very weak ability to poke through waves, because it will begin to retract as soon as it hits something. His AA poke may be unavoidable, but with his short AA range he becomes much easier to engage upon when he tries to use his AAs to poke.
In mega form, Gnar's abliites are heavily telegraphed. This is his biggest weakness. While it may be hard to dodge a Q point blank, dodging one from range gives you the ultimate opportunity to engage upon him while the majority of his damage and one of his CCs is on an extended cooldown. Without the slow from Q, sidestepping his W and E become easy.
I'm not saying he's not broken or anything - his high banrate does seem to suggest there is something wrong with him. I'm just offering some advice on how to better play against him.