It might be best to have a central place for discussion of Gnar balance, and get it all in one place instead of having topics all over the place.
Anyway. After playing him a bit, he seems a little on the strong side, although a lot of that might be due to the opponents not really knowing how to play around his passive. Few key things I want to mention.
* His boomerang feels kind of weak, damage wise. In laning, and in teamfight poke wars. The base damage of it could stand to be increased a bit, I think.
* Mini-W feels fairly okay. I can notice it's impact in fights, but his range makes it a little hard(which is fine, but means it's not really usable as a primary harass tool).
* His E seems to have the double hop when he uses it while transforming. Not sure if that's intended, but it would be nice if that could stay in. It feels REALLY good using it for an initiate, since it lets you get behind the enemy in Megagnar form, and is telegraphed in advance. Can be easily countered/knocked out of/so on.
And then there's his bruiser form! The passive works as intended for the most part. Do feel like it's something that HAS to happen every now and then. It's hard to manage rage and not pop into your passive before a teamfight, but not impossible. Feels bad to pass up a few minions because you might need to transform in an upcoming fight, though, but workable. Few more points...
* Q in megagnar form! Lots of fun. Really feels like I'm smacking a person with a giant rock. The sprite looks sort of out of place, but it seems like it would go fine on the summoner's rift rework. Of note is that, I'm not sure, but I think I picked up another megagnar's rock before. Weird.
* W is... good. I like that it has a delay on it. Might be a bit too much power though, bringing me to my next point...
* His bruiser form feels strong. Really really strong. A lot of fights(1v1, granted) felt really easy for me when I got next to them, and could just chain stun them with R, into W, into Q, and so on. He just did so much damage while chain CCing them and locking them down. Kinda like Thresh. I love his R though, and it's really fun in it's current incarnation, but I think the CC or damage needs to be toned down a bit.
(EDIT: Whoops. Already is 1.5/1.75/2. I fail at reading tooltips, nevermind! ) It also seems almost required to level up Q first because of how important it is(20 seconds to 10 second cooldown!), with his other stuff feeling... not as important to level, especially his E.
In conclusion, he is a LOT of fun mechanics wise. And absolutely adorable, too :3