Hi, i'm Cpt.Dodo from SEA so please forgive me my english mistakes.I played gnar so much in normal and ranked.
I will talk about the best, non trolling way to build him, which is full tank. I'll explain why everything else is not realistic for serious games. Be sure to review and comment my guide if you think it is interesting and deserves improvements
I bet you anything first time you played gnar you build full ad but you wrong why because what is hurting on gnar is the W passive Hyper , and this spell screw up your entire perception of the champion.The good mindset for building gnar is to think about something that will support both of his forms . Tankyness is clearly good for mega form , but he will also supply the baby form because you can't effectively trade in the top lane without resist , you'll just blow up to ANY combo . However after you bought some resist to survive that burst , you'll be able to trade as your ennemies try to disengage after their combo with your range and Hyper.
Time to try my guide and i think you can like gnar now :D