gragases barrels are a tricky one, for some reason sometimes the aoe seems smaller or larger then it actually is
i play a few champs and for a while i played gragas extencivly , a banshees screws him over right and true, but over all to stop gragas you have to remove either his sustain (eg mrs fortunes shots that reduce healing) or his damage, dodgeing his barrels isnt a guarenteed method as a good gragas player will ulti you into his barrel (assuming laneing phase and not team fight phase) or put it in your escape route once your low
overall gragas is strong yes, but remeber he's just a bully and newer gragas can easily be out bullied by any ranged ap champ but if your dealing with a good gragas, a gank might help ALTHOUGHT gragas is hard to gank due to his short cooldown dash and his relative bulk, it can still knock him out of lane for a bit and once you get ahead in farm and xp it becomes easyier to bully him, but never think youve won the fight
an underleveled gragas can still burst you hard with his standard combo