As Gragas you should take blue buff as often as possible without disturbing the allied jungle path. This will allow you to get the upper hand on the enemy mid-lane whether or not you are still in the laning phase. Taking jungle creeps with Gragas is easy, simply roll a barrel(Q) into a jungle camp and bodyslam in or over the wall and detonate your barrel, this should allow you to farm easily especially to nearby wraiths.

Gragas excels in farming in most lanes as his area of effect is very high, and as he is relatively tanky compared to most AP characters it is difficult to harass him. If under constant attack and in need to fight back, use your Drunken Rage(W) buff, then charge at him with Body Slam(E), then Roll your barrel(Q), this should deal a high amount of damage and make them aware of Gragas' ability. FIRST BLOOD GG

Gragas has good ganking potential if done correctly, simply hide in the brush waiting for the right moment use your Big Barrel(R), knocking them back into your turret and body slam(E) into them if your team can not finish them after this YOU NEED A NEW TEAM. Gragas is possibly one of the hardest champions to pin down and gank, as you can simply Body Slam the F*** out of there, if Body Slam is not enough you can use your ultimate to knock enemies out of the way. Even though your ultimate is a great defence mechanism it is not ideal for escaping as one simple miscalculation can cause you to die.

Explosive Cask(R) is mainly used as a support mechanism in team fights to spread the enemies, but can also be used to last hit escaping enemies and knock targets towards you for an easy kill. Explosive cask is a skill shot nuke that does a high amount of damage to all enemies in the Area Of Effect(AOE) whilst also knocking them back depending on their position in relation to the center of the AOE. CAUTION!!!!!! Explosive Cask whilst allowing your team to easily win fights, can also cost you vital kills and help enemies escape, so assure that you get the right timing and aim on the targets.

COUNTER GRAGAS???? IMPOSIBRU!!!
During all the time I have played Gragas i have had quite a bit of trouble with laning in mid againts most AD carries. AD carries can counter darius and take advantage of the fact that he is and Ability Power based Champion and bound to be squishy. Gragas is known well for getting in range dealing tons of damage and getting out, AD carries will deal tons of damage while gragas is attempting to poke and come into range.