This is the best build because when you are mini Gnar you get the crits for autoattacks which are faster in little and it does a LOT of damage if it critical hits when the 3rd stack of Hyper(W passive). Plus, if you activate Youmuu's Ghostblade's item it will stack attack speed buffs and mov speed buffs, so you can run, chase and engage better. When you evolve, mega Gnar will be able to do a LOT of damage too, yes with capital letters again, with everyskill because you will have great damage coming from Infinity Edge, Youmuu's Ghostblade armor pen, 30attack damage(it's something) cooldownreduction which can make you spam your Spells, the active does better on mini Gnar. Frozen Mallet does nice damage too , applies slow, and has 700+ HP. Atma's Impaler is waay too op: It gives 15% crit chance(which stays in 60% crit chance(overall) with infinity edge and youmuu's) 45 armor!! and the god given 1.5% of Max HP for attack damagee. So when you are mini Gnar you have ~306 AD and when you are mega Gnar you have ~340 ( ~ = around ), at full build. Spirit visage increases greatly well your HP regen and its passive is great, I dunno if it augments the numbers of HP on mega Gnar, if so it is great. Also it gives magic resist, and 10% Cooldown Reduction.

The main thing to do with Gnar is, to have pacience. You need to strike at the right times. But overall, always catch your boomerang. Little Gnar's W passive is Great so be sure to use it always on your opponent. Try to poke him with it - 3 W's on enemy then back. Remember that E and Q stack W's as well; only use E, when little, ON your target if you are sure you are gonna kill him and if he is going to escape or if you just want to mess around. Poking with Q is also very very important, when you throw your Q you can hit the enemy once AFTER cathing the boomerang and then, when Q is ready again, use it for W passive. Transform and engage only when you know you are going to have the advantage, if you are not sure, don't do it. See at where they are going and not where they are, you need to calculate/predict your enemies' movement/location as Mega Gnar, so you can hit your spells. As for the ultimate, try to use it always near the wall for the extra damage AND stun. GNAR!

I wish I was helpful :)