To be honest, they don't like AP Gragas for a reason. With the kit he has now, AP Gragas is actually pretty binary. He is either unable to wave clear and too squishy to go in for kills, or he is able to wave clear and his burst is over the top.
Right now AP garages is workable, if a little weak. He has solid ratios across the board (up to 250% AP across a full burst rotation) And pretty good base damage (1180+8% of the target's max health) and enough CC that he can usually burst a single target without retaliation (slow, 2 knockbacks, and a stun). Throw in the fact that any good AP Gragas builds Lich bane for an extra 91 +(50% AP) damage at level 18 and he's actually very bursty.
Only because of his Q taking time to charge to max damage and his reliance on now getting up close and personal to deal his burst is he less favorable. Personally, I think AP Bruiser Gragas is pretty strong. He has good ratios and all you need is a Lich bane and maybe an Abyssal and then just go tanky (maybe build a RoA if you are ahead too).
EDIT: Really, looking at the video he is actually just fine right now. All you have to do is set up your own combos and his burst looks fine. You're asking for buffs for a champ right alongside a video showing current footage (at least since the last time AP Gragas ever received nerfs) of AP Gragas stomping people. That is the largest argument against a buff that I can think of.
