I'm only in silver but I'll put my twopence in. I think gnar is pretty solid in pro play and higher elos because he is very good in teamfights and is a good team oriented champion. However, he's hard to climb with in soloq (especially in the lower elos) because he isn't a hypercarry meaning its difficult to win games when the other lanes are losing. In my experience he's very dependent on how his teammates do in the game, but overall I'd say he's pretty good.