Basically you just gotta respect his high range and flash (he will oneshot you). If you can't outrange him in lane, you can either go for an all in early while his CDs are still high, using his melee-ness to win fights with minion aggro, or you can farm up and go teamfight. AP Gragas isn't great at teamfighting since he's so squishy, but he'll ruin you if your scattered. Tank Gragas is never really bad but hopefully they don't have enough damage on your team, so just body block his E in teamfights and be careful of his R. It's pretty impossible to win lane though.