Kass won't win. His pre-6 would be so terrible that he can't recover by laning mid if the gragas is decent.Denying every, or 70% if not all cs would make it real hard to kill gragas without compatible items. His only chance will be roaming other lanes and hopefully getting some kills cause that's his only main gold source.
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Does he fall off late game? No, not really. Get yourself a void staff and dont use body slam to run towards a group of enemies and you'll be just fine.
As for his counters... To be honest i havent lost my lane when playing Gragas, but im guessing champions with mobility can avoid his barrel pretty well, but you'll still be able to farm safely with it because of its long range. Good example of this is Teemo. Sure, he's really squishy but as long as he keeps moveing its pretty hard to hit him with your barrel. Guess you could try W->E and then a point-blank Q to his face. Also Kassadin can probably win the lane post lvl 6 as he can just jump around with his R.