Gragas is one of the best lane harassing champions in the game. Gragas is one of the best mids against a ranged carry in my opinion. Every time I've gone mid as Gragas I end up getting a kill at level 6. The ranged carries typically have no escape method once you ult them back on or behind you. If they have flash then they flash and possibly get away, but you still end up the winner because you get to push mid and gain experience while they back. If you've never tried mid as Gragas, I would suggest it.

Rod of Ages is a must have for Gragas as soon as possibly so that you can start getting those passive bonuses. Rod of Ages and Rylai's Crystal Sceptor are amazing with Gragas because they give the 80 AP but they also make him very tanky. Lich Bane is a good thing to end with as Gragas for that bonus when you use an ability. You can also change an item for Void Staff or Morello's Evil Tome. It is up to your preference.

Pros:
great lane harasser
great aoe damage
good against melee ad champs with attack speed debuff on barrels
ult that can screw over the enemies or save an allies life if used correctly
amazing lane sustain with passive and his drunken rage

Cons:
if you don't place your barrels good then they're easily dodged
ultimate can often ruin a chase or fight
hard to recover with