Well, Gnar is just like Teemo or even worse, be annoying in your lane then come into mid-late game and have no impact on the game. You either make yourself tanky or try to go for some sort of full damage build, together with his passive being a bit random he will never really become any sort of a big champion. As a toplaner he can't even compete with someone as Alistar/M a okai even someone like Swain which is a very popular top pick nowadays. As a adc his passive makes it nearly impossible to become a good ADC.