Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
After you finish the build and if u think you can play Gragas well you can sell your boots and buy a deathfire Grasp for more damage but it's not needed if the enemy's team doesn't a tank or a fighter with much health.
Barrel Roll - Br
Drunken Rage - Dr
Body Slam - Bs
Explosive Cask - Ec
Playing as Gragas is important to always harass with Br and use Dr as fast is off cd.
A good tip is using Ec on enemy to push them in to a friendly turret.
Ec is also good to save in fight to push them back to you while they are running away.