I just think of Gragas as a very SAFE champion. He can't be punished so hard for making a bad move because he has an AOE slow, a stun/dash, and a huge knockback if he gets into any bad situations. His Ultimate is really strong as a displacement to separate the enemy team and pick them off one at a time, but it can also stop your team from securing a kill or end up killing your carries if you use it incorrectly.
As far as his damage, his only source of lategame damage is his W, everything else is negligible for anyone besides really squishy targets.
He has early game pressure unmatched by any other tank in the game (Zac is the only one I think can do as well as him), and he scales pretty well into the lategame, so he is a really strong and very safe pick as he is not reliant on snowballing. Rek'sai seems to be his only real equal in terms of exerting pressure early and not falling off very hard in the lategame (though this is mostly due to her Utility and map presence versus Grag's CC and Ult). Other tanks heavily out damage him in the lategame, but they usually have really weak early games;
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I wouldn't mind if he got nerfed some more, mostly so people would stop banning him and I could play him. But I really hope if they do any meaningful damage nerfs to his kit, they at least do something about his laning. Right now, it's pretty shit. His AP ratios were all nerfed, but I think that was warranted. What I really found hurt him in the laning phase was the damage reduction to minions on his Q. Basically, if you nerf his damage, his laning gets even worse and he is forced into Jungle or Support.
Before Cinderhulk, Gragas had a 45-46% steady winrate in soloqueue. He was mostly played mid and top with occasional jungle play. But his winrate was a reflection of how hard his laning was hit by the damage reduction to minions on his Q. I think it should be reverted if he receives any nerfs to his base damages.
Anyway. Those are my thoughts on the matter. I don't think Gragas is as broken as everyone says he is, I just think he is incredibly safe and hard to punish. I really wouldn't mind if he got nerfed because it would shut people up about him and maybe I would be able to play him again (he's fun to play). We'll see what happens. The new AP jungler item is going to be released soon and we'll see if that pushes some junglers into favor (probably won't happen in its current state: 40AP?). Peace.