Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
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)