I see a lot of people jumping on Gragas and playing AP since dreamhack. But everyone is wrong. You want to play AD. Seriously. You'll see why later.
These runes are great. Your early game consists of charging in and murdering them from your 102 (or more) damage at level 1, and minions are not fond of you beating on their champs, so you should get some armor. The AP runes are because he does get decent scaling on his E, and early game it will be worth it.
I took the most common AD carry runes in the game. Can be replaced with beefy DPS masteries (and I guess runes) if you want, but that's not how I roll.
The early Avarice Blade is necessary, and you should really get first blood, so it sets you way ahead of everyone else as far as gold goes. Everything else is so you can chase indefinitely and wreck their armor for more basic attacks. Boots can Last Whisper can be replaced with spell pen, since E is still magic damage. Furthermore, the CDR from youmuu's will make you able to keep the Drunken Rage buff up perpetually.
Skill sequence is obvious. You want that damage from Drunken Rage, but body slam is where your real damage comes from until it's around black cleaver time.
Also obvious. You have a dash, so with flash you can suddenly get across 2 walls in a second. INSTANT ESCAPE. Exhaust is to lower their armor for an easy first blood.
Really, all you should do is get first blood and Avarice Blade, then farm. Farm and farm and farm. Kill people when they underestimate you and then go back to farming. Nobody will think AD Gragas will be good, take advantage of it.