How to play Gragas in ARAM

1. Most people spam their Barrel Roll and rarely hit anything because it moves very slowly. You should use the ability to let it explode later, the same ability Lux has, they are both great at zoning. Place your barrels next to the enemy tower so that they can't pass the small gap, this gives a lot of utility to your team. Never spam your barrels somewhere enemies stand at the moment, zone your enemies, they mostly have to go through it anyway.
2. Get cooldown reduction early, it's more important than damage. Get Athene's Unholy Grail and Spirit Visage for a little more than the CDR cap (44%). When you reach that point in the game you can constantly roll barrels.
3. Make use of Gragas' passive Happy Hour, spam abilities whenever you can to heal yourself. This heal will grow bigger with Spirit Visage while you also get tankyness and flat HP regeneration.
4. Itemization: Rod of Ages is an okay item but you rarely get into late game, you also don't have mana problems because you already have Athene's Unholy Grail and Drunken Rage. I replace it with Spirit Visage and very offensive and high AP items. This way Gragas gets an extra 20% CDR, generally higher damage early game and high amount of health regen.