Okay this is my first build so I'm not to sure about some of this stuff...
The main idea behind this build is the items. To me the items are what it's all about.
I usually start with a ruby crystal to have a edge on the other champs hp wise if we engage before minions spawn. Not to long into the game(once you get 500 gold) you can get Heart Of Gold, which is a standard tank item for me.

The next item you get depends on your enemies. If your fighting primarily magic users I would go for the Spirit Visage. Eventually you'll wanna turn that Heart of Gold into Randuin's Omen, which has an awesome ability for chasing enemy champs. To me the sunfire cape is just for good measure (it is really good at killing minions fast).
I go Mercury's Treds for boots usually because i like the defensive type specs on it, but you could do Boots of swiftness or mobility i dunno.
I actually sometimes don't even get boots with Gragas because his body slam is so useful for getting away. Remember you can body slam through a lot of different walls it just takes some getting use too. To me body slam is by far Gragas' best move. It's good for attacking/chasing(it slows!), and it is good for running away(go through walls!).
Important Tips

Don't always use your barrels, your ulti is good for hitting champs into you for a kill, but they do use quite a bit of mana.
Use

drunken rage before u have a team fight or attack a turret (the attack/defense buff is awesome!).
Remember to buy mana potions every once and a while (Gragas has really good HP regen but not so good MP regen except for drunken rage)
Body slam has a low cooldown so spam that, it's fun! (:

The next item you get depends on your enemies. If your fighting primarily magic users I would go for the Spirit Visage. Eventually you'll wanna turn that Heart of Gold into Randuin's Omen, which has an awesome ability for chasing enemy champs. To me the sunfire cape is just for good measure (it is really good at killing minions fast).
I go Mercury's Treds for boots usually because i like the defensive type specs on it, but you could do Boots of swiftness or mobility i dunno.
I actually sometimes don't even get boots with Gragas because his body slam is so useful for getting away. Remember you can body slam through a lot of different walls it just takes some getting use too. To me body slam is by far Gragas' best move. It's good for attacking/chasing(it slows!), and it is good for running away(go through walls!).
Important Tips