Alright so, gnar, played him for a good 30-35 games now, not counting the ones on PBE, and since I mained top lane for quite some time now, I compared him with other champions/matchups I have experience with in the role, and here's what I got (keep in mind that people still don't really know how to play against him, so this won't be that reliable) :

Early game/laning phase is a breeze. Pre-6 should be extremely easy, you have more than enough utility to survive ganks. You have one of the most annoying pokes in the game, pretty big slow and an immensely low cooldown on the Q, and being manaless, makes it way too frustrating for the enemy to keep an eye on their cs and still managing to dodge. Your AA range isn't too good, and will draw minion aggro when you use it to poke even if you're ranged, so I wouldn't recommend doing that when too many enemy minions are nearby. Save the AAs to proc the passive (remember that your abilities add stacks as well), and to poke when not too many minions are around.
Be careful not to double jump into the enemy turret, I've done that too many times now just cause I neglected minions' existence.

Level 6 is when your burst really shines. Poke poke poke until your rage bar is filled, double jump while transforming, stun with both ult and W (chain them, it's a 2.5 second stun if no tenacity) and you would be dealing immense damage with little to no items.

Itemization should always start with a trinity. Seriously, stop trying to start botrk.
Trinity > brut > botrk > ghost blade > tanky item/lw > lw/tanky item should make you hugely frustrating to deal with. Might wanna replace lw/ghost blade with a tanky item if you feel you're being one shot too often.

You will do amazing by split pushing, and a bit less amazing team fighting, let's talk about both situations :
With the build mentioned above, split pushing is fast, efficient, and will almost always require more than one enemy champion to deal with you, which gets pressure off your team so they'd be able to take objectives (which, actually is your job as a top laner even in laning phase and mid game). If you know you can fight someone and kill them fast enough when they try to defend from your split pushing, go for it, but if you're not so certain, just run away. Your late game Q will have it's cooldown go down to 0 when you're throwing it backwards into enemies and catching it. And your W proc is brutal... BRUTAL!

Now for team fighting :
Right now, I have yet to do spectacularly in teamfights unless all the team's ahead. Because unless you're in the jungle with walls around you which gives your ult amazing team fighting power, you're gonna have trouble using it. So if that doesn't happen, either you'll throw them all into your team, throw them all away, catch a couple people to stun, or worst of all, switch back to mini gnar and become a short range adc for 13 seconds + the time it takes you to fill up your bar again.

But don't forget the double jump into transformation when your rage bar is full, cause that gives a pretty great initiation if you can position your ult right. Look to choose gnar into comps that have nice aoe cc, like amumu, leona, ori etc...

I remember one game, we were very much behind, someone got caught in the bottom lane next to the wall, they managed to survive thanks to a 4 man leona ult into my gnar ult to the wall, the damage from that ult is absurd and if you chain the stun with your W on them all, that'll become an ideal teamfight.

So yeah, this guy needs practice, he's a whole lot of fun to play, but it's hard to make him very relevant outside laning phase and split pushing.