So coming from a Gnar main with about 400+ games on him, I understand perfectly that he can be very difficult to counter if the Gnar in question is halfway decent. With the exception of Irelia, and maybe Yasuo, I thoroughly know how to beat all of his supposed counters in a matchup. I'm not going to tell you he's easy to beat, because if the Gnar knows what he's doing, the lane can be a nightmare even with his counters.
I doubt there's much I can say to convince you that he's not the optimal top laner, but there's two important things to consider.
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Gnar with his two forms is essentially a jack-of-all-trades but master-of-none. Mini Gnar is a duelist who specializes in 1v1s through kiting with his Q slow, W MS buff, and E escape. The counter to him is essentially the counter to any ranged, kiting champion, collapse on him. Now this made more difficult because of his Mega Gnar form, which is designed for massive AOE CC and burst. These two forms would be seemingly impossible to balance with one another if it weren't for the caveat to this seemingly awesome power...
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He has no control over it. Your entire presence as Gnar revolves around being able to engage on the fly with your E into Mega form and disengage once you're 19 seconds is up. The thing is, any damage you take, or give will fill up the Gnar bar. That means that if you're sitting at 90/100 and you're hit by a few minions or an Ezreal Q or what have you, you're going to transform, and if you're not set up to engage, you're going to blow your chance to be useful in teamfights. That in my opinion is Gnar's biggest weakness is that it's so easy to whiff up your engage with Mega form, that maximizing your efficiency with these two forms can be a roll of the dice.
With that being said, that's why picking Gnar just haphazardly can be a risky thing. For that kind of engage, you could just pick Sion and do what you're supposed to do consistently with little RNG about it. Anyway that's just my two cents. I understand if I haven't completely changed your mind, but I just thought I'd give you a perspective from someone who plays him a lot.
and Q. In tank form he walks all over you with massive AoE damage and DOUBLE STUNS. Gnar right now has got to be the most dominant tank and top laners in the game right now and I honestly don't know why he doesn't see any attention. Absolutely no reason to just not play Gnar when going top. His only "counter" is Irelia.
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