Not Updated For Current Season
-Ignite, used to be clarity but I've learned to play with out.
-Flash, not needed for the build just a personal preference.
-Great at lanes
-Barrels are amazing harassment
-Body Slam through walls
-Regen health with happy hour, this + body slams short cd works well
-Can pick off champions with ult and stop melee focus fire
-Body Slam is great for chases/escapes
-You are Gragas your a huge target! People can easily click on you for abilities unlike say a Teemo
-Ult is hard to aim and can easily miss fire
-You go oom fast with this spec, but 5's mana buff makes it amazing
-Early Game: Gragas is amazing early game, the early specced barrels do a ton of damage. Lane with a good harasser. When playing with my friends I generally lane with a pantheon and I pick up Improved Clarity and take 1 point out of expanded mind. With this build you will run out of mana fast make sure to use drunken rage every cooldown. I usually pop clarity around level 2 and use it on CD from there on. Gragas can solo lane and is very good for 2v1 lanes if you have a jungler. I generally can push back and exp deny in 2v1 situations or get an early game with my ult up. If you are soling a lane pick up Doran's Ring for the mana regen and constant harassment. At level 3 I can usually get a kill from previous harassment and an unexpected body slam.
(Side Note): If you can keep the stacks make sure to grab the Mejai's Soulstealer around early mid game, if you can't keep the stanks don't waste your gold.
-Mid Game: When you first start team pushing around level 10-13 turret diving can still be a bit rough. This is where your Ult comes into play, ult ppl towards you away from towers to pick off chapions before team fights it can help out a lot. If a tower has low health but there are a few champions and around it playing D knock them all back and push the tower down before they recover. Don't initiate team fights you may be a tank champion but you are mage specced you will die fast.
-Late Game: Unless the other team has been fed hard core your barrels should be hitting rather hard at this point. Constantly harass the champions before the team fight is initiated.
(always start by using Drunken Rage)
-Charge + Barrel + Charge (you may auto attack for a second but with charge's short cd this is a decent amount of burst)
-Ult + Charge + Barrel + Charge (they just got raped)
Chasing and being chased:
-Chasing: Use your charge first to slow, if not they can out run you if you stop to throw a barrel. Once slowed barrel in front of them if they change paths to avoid it that is more ground you can gain on them. If your ult is up slow them than ult them into you.
-Fleeing: Body Slam constantly to gain ground. Try body slamming through a near by wall to become unchaseable. If its one champion and you can kite them long enough constantly drop barrel and explode them on the champion I've turned a few chases around this way and got a kill or two. If you can drop barrels so they avoid them and you gain ground while they can't run in straight lines.
(Side Note): Avoid people who can snare like Ryze and Morgana you can not body slam out of this and will most likely die with out help.
-Watch where people run to avoid your barrels throw them there. Generally backwards but can be to the right or the left.
-If your doing it right barrels start to one shot minions making it very easy to farm gold. I never recall to buy items under 750 gold.
-Boots are very dependent on the other team look and their comp and decide what to buy.