Gragas is a very fun champion to play. He has a spammable "Q" that can do massive damage, his drunken rage gives nice damage reduction, attack power, and helps with mana issues to keep you in lane longer. His "E" is great for slowing champs down, but i think the most useful thing about it is that its a great escape tool. You can even jump over thin walls to either get away, or to surprise champs who arent expecting it. His ultimate is great too as not only does it do massive amounts of AOE damage, but it can throw their whole team out of position, or throw champs into towers, or single them out to allow your team to take them down. This also is a great escape tool because you can blow champs away that are trying to chase you down.

I use gragas with magic penetration marks, flat armor seals, mana regen/lvl glyphs, and ability power quintessences. the magic pen and ability power runes are kind of a given, considering you want to do max damage with your barrels to harass. The armor seals give you some survivability to stay in lane longer, and the mana regen glyphs do much of the same, helping to keep your mana up so you can spam your barrels to keep up the harass.

I build gragas around the 21/0/9 mastery scheme. This allows for maximum damage from your barrels, while the mana regen from the utility tree allows you to stay in lane for long periods of time without having to return to your base.

With gragas i always take ghost and exhaust. Ghost is nice because it helps you to get away and chase. Exhaust slows enemy champs down and reduces their magic resist, helping you to land that last hit barrel. It is also good when a strong ad champ is attacking you, as it will slow them down and reduce their damage allowing you to get away.