So, overall he's a VERY fun champ to play. And standing in a bush /danicing drives your opponent mad.
As far as skills go, here's my breakdown:
Passive - Not much to say. You gain tantrum which turns you into a big hulking monster, and you get minor stat boosts for 15 seconds when you transform.
(Small): Ok this needs some tweaking. 10+ 1.0 AD ratio at rank 1? Granted it does jump to 45+Ratio at rank 2 but come on. It's useless as poke aside from getting a stack of W. It only deals full damage to the first enemy hit, and half damage to any other enemy, with no rebound. And on top of that, if you miss catching it, it goes on a long CD. It either needs a damage boost, or do full damage to all enemies, with a rebound of 50% damage.
(Big): Better than Small Gnar's. It atleast does AoE damage to anything caught in the hitbox. Again, could benefit from a small damage increase.
W: I actually have no complaints about this skill, in either form. It's at the current time his main source of damage in small form, and has sticking power in big form.
(Small): Dear dark god teemo I love this skill. It's Fizz trollpole all over again. It's a nice was to escape jungle ganks, and to gank by jumping over walls. Also nice to jump off your teammates heads just because.
(Big): Short range gap closer with some damage attached. No real complaints.
(Small): Pretty useless all in all. I'll explain why in the Big Gnar section.
(Big): Ok, so you have a champ that turns into a hulking monster that beats the ever loving crap out of things. So why is it so scripted when he's going to change? 'Oh look, hes near full bar. Time to retreat for a bit.' No gap closer or speed boost is going to change this. The only way he's reliably going to get his ult off is if he transforms in the middle of a fight or jumps out from the FoW. Make his Tantrum bar no visible to enemies. He already has visual cues. This just makes it to where he can't use his full kit in laning phase, and let's the enemy know quite clearly when to go in on the now very squishy small Gnar. Something needs to be changed so enemies don't have a year's warning.