Well what I think will be good for Gragas, is Lich Bane, Hextech Gunblade and such. The Lich Bane will proc giving Bodyslam good damage. Builder. With that build I think is one of the few cases Manamune is good for (I hate this item otherwise and think it's useless).
The builds a tad expensive, so I say rush Lich Bane, and go from there.