I've been playing quite a lot of Gnar recently since I find him lots of fun, trying to take care of his passive and everything. None is left unsurprised once I jump on them in Mega form and I knock them back on the wall. I tried a couple of builds on him but I think it's always worth to try a new one.
The first item I build on him 90% of the time is Trinity Force. Phage gives me a real sweet speed to keep throwing AA's on my enemy until I proc my passive and repeat that circle again. Sheen helps me mostly in my Mega form. I use an ability, AA, another one, AA and so on. So I find Trinity Force a real strong item for him.
I never tried it so far but I'm wondering if Frozen Mallet is a viable second item to build on him considering the slow it gives to the enemy which will make it easier for me to keep up with the enemy and keep auto attacking him until I proc my passive on W and repeat that process with the speed boost I'll get.
You might tell me that I focus too much on my Mini form while in teamfights I should be mostly in my Mega form. Well, those are only the two first items. They will hopefully help me in the laning phase, take some kills and get ahead. That's my thought behind them.