This is my first guide on www.mobafire.com, so please be nice :)
Try the build before voting up/down, and leave a comment if i missed something.

Okai, so let's start off with the runes.

Greater Mark of Magic Penetration for magic penetration, so you got more damage early game.
Greater Seal of Ability Power for ability power, also gives good damage early and late game.
Greater Gylph of Potency for ability power, also gives good damage early and late game.
If you are having alot of mana problems, you can go for Greater Gylph of Replenishment
Greater Quintessence of Health, for magic penetration, so you got more damage early game.
Greater Quintessence of Ability Power, for ability power, also gives good damage early and late game.

As you can see i go for flat AP and MP for early game domination.

On masteries i go 21/0/9 and pick Ignite and Ghost.
I go 21 on offence for best damage possible.
9 in utility and pick as much mana and mana regen as possible. And i take Greed for the extra gold.

on items i start off with Doran's Ring.

On your first trip back (you should have around 850-1000) buy Boots and an Doran's Ring, if needed take 1-2 Health Potion. And don't forget to buy Sight Ward ! IMPORTANT to avoid ganks!

On your second trip back, upgrad your boots to Sorcerer's Shoes. If you don't do well and need health pots, buy 1-2 more, and again 1 Sight Ward.

Now you should have 2 Doran's Ring and Sorcerer's Shoes. And it's time to get some ability power for extra dominating. Go back to your lane and farm, maybe get some kills ;)

Now on your next trip back, you should get Needlessly Large Rod for your Rabadon's Deathcap.

When you got your Rabadon's Deathcap, it depends on how your doing what you get next. If the enemy team is doing alot of damage, take Rylai's Crystal Scepter or Guardians Angel. or if your enemy got a tanky team, go for Void Staff.
But normaly i go for Rylai's Crystal Scepter, it gives you nice HP and AP, and got a nice passive.

But it's really up to the situation in the game.

Give you some nice AP and mana regen, and some cooldown reduction.. very nice. Aswell as a nice active against tanky champions.

- Gives you abit of everything you need. And gives some HP, mana, and AP every 1 minute.

- This is a good item to get if you are getting alot of kills/assists. But you'll lose a third of your stacks from it, if you die. So it's risky to take this item.

- Get this item if you are playing against AP burst damage champions, like Le Blanc Veigar Annie.

- Playing against a team with alot of magic resist ? Get this item.

- Usefull if you are playing against AD champions, like Tryndamere Caitlyn. Or if they got Karthus, use the active of this item.

- You get good mana, HP and magic resist. And a VERY nice passive.

Barrel Roll is your main damage output, so pick it whenever you can.
Use this spell to harras your enemy. And clear the minions away from your path.

Drunken Rage is a very, VERY, important skill. Max it after your Barrel Roll
Try to always keep it up, because it reduces incoming damage, and gives you a little attack damage. And the most important thing with this skill, it gives you mana while channeling. So you can harrass alot without getting mana problems. BUT if you harrass too much, you will still get mana problems. But after awhile you know how much you can harrass and still keep your mana up.

Body Slam a very nice skill. Charging forward and colliding with the first enemy, and damages all nearby enemys, aswell as slowing enemys hit. you can also use it to charge through walls for escaping/chasing. When using this along with Rylai's Crystal Scepter you will slow your enemy alot.

Explosive Cask Really nice ultimate with a BIIIGG explosive location. Damaging enemys and knocking them back, really nice skill to use to get enemys away from their tower, or to your tower. And with it's low cooldown, never be afraid to use it.

In Summoner Spells i pick Ignite and Ghost.

Ignite Is a nice spell to get the few last healths down, or you can use it when playing against Warwick or some other champions who got lifesteal or healing spells, to lower their healing.

Ghost Is a good escaping/chasing spell.

Other summoner spell you could get:

Teleport Is a nice summoner spell to get if you are solo top or mid.

Flash Nice summoner to get instead Ghost

Exhaust I don't like this spell but it is viable.

Summoner Spells not to get:

Clarity You got your Drunken Rage. Don't waste your summoner on this.

Heal Have you even read your passive ? just NO.

Clairvoyance Leave this to your support.

Rally Really ? this spell is nothing but USELESS.

Fortify Leave it to your tank.

Revive No. Just NO. you are the killing machine.

Smite You ain't jungling.

Gragas works on every lane.

Solo Top is where i play gragas (when possible).

When i play Gragas i play abit defensive. Stay back and throw some barrels at the enemy. and use your Drunken Rage whenever possible, to avoid getting mana problems. When you have harrassed your enemy and they got little hp, use your Explosive Clask to get them over to you> Body Slam to slow them, so they can't escape> Barrell Roll for some damage and ignite if needed. By this combo they should most likely be dead. This is specially good if the enemy is pushing alot, and you use your ulti to get them over to the tower. Then it's a 100% kill.

So yeah.. overall, i play Gragas pretty defensive and harrass, until i reach level 6, then do my combo (W>R>E>Q>ignite if needed), and it is a pretty sure kill :)

!!NOTICE!!: If the enemy got a tanky top lane (singed, galio etc.) ask your team to take someone for top, and go mid. (use the same build and such for mid).

25/08-2011
Added [Situational Items] section.
Added Sheen/ Lich Bane to Situational Items.
Added Rod of Ages to Situational Items.
Added Mejai's Soulstealer to Situational Items.
Added Force of Nature to Situational Items.
Added Abyssal Mask to Situational Items.
Added Zhonya's Hourglass to Situational Items.

If there is something you would like me to add, just write it in the comment section, and i will test it out :)

There really isn't much to update here. This still works great.