after hearing a lot of friend advice and reading guides, i'm really clueless. i've seen guides that list RoA as a "must have" yet they don't put them in the build order on the top (mobafire etc) and i've heard people to tell me to rush a needless and a bunch of other different things. i'm not quite sure what to do on him anymore except for "get ap items" --- http://blackacrebrewing.com/hey.swf DotA 2 random player
RoA is insane on Gragas. He has percent damage reduction which synergizes very well with the health it provides, not to mention Catalyst is a super early item for him.
But yeah, he's pretty much built like a bog standard AP Carry, but you can get away with building slightly tankier so you can throw out more barrels in a teamfight.
My standard build is: Sorcs RoA Abyssal Zhonya's Rabadon's Situational
With that build you'll be doing trash for damage for the longest time. You need double dorans into Deathcap on gragas, and if you're really farming that well/fed. Get abyssal or spell vamp first, then Deathcap.