Gragas is a champion that many... misunderstand. He can do a lot of damage while knocking people around and also slowing them.

Normally I go with strait AP runes, the build will make you beefy enough to be able to take some punishment. You'll need the extra damage.

Start off with a Dorans Ring, early game its will give you a big advantage for damage and harass with out losing all of your mana. Spell Penn. boots, in case they get a bit of magic resist. Rod of ages for more mana and hp. A Rylais next for slows, AP, and more hp. Aster the Rylais, it depends on the team. You should get a force of nature if magic is hurting you, or a sunfire cape for more hp and armor or thornmail for armor. The last item will be a Atmas Impaler, this gives you more AD for the bely flop, (Which scales with AP and AD) and give you armor for the survivability. (Sell Dorans for Atmas)