Playing Gragas competitively is absolutely amazing, and a well farmed Gragas is going to make or break a lot of team fights. When I play with this build I usually get compliments about the damage output and people asking if I stack Magic Pen runes etc.

Give it a shot, I guarantee you won't be disappointed.

Pros:
-Amazing lane sustainability
-Amazing poker early
-Big damage ALL game
-Can be the deciding factor in a team fight

Cons:
-You'll be targeted with everything they have
-You can be taken down quickly

I go with a very basic AP caster rune selection.

Magic Penetration marks (These can cancel out any characters base magic resist)
Greater Seal of ReplenishmentMana regen/level seals (This is going to make a huge difference early and is going to allow you to start with boots without running out of mana)
AP/level glyphs (These let you keep killing caster minions with one barrel)
Flat AP Quints (Gives you strong damage early game)

I go with a basic 9/0/21 which works well for this Gragas build as you're playing an AP caster role.

Starting
If you're going to get poked:

If you won't take much damage early:


First trip back:
Getting boots early is going to make a HUGE difference. With Gragas in mid you can gank either top or bottom really quickly by using your E to get around. Boots help you out and will also cancel out all default magic resist on most champions.
Sorcerers Boots

Second/Third Trip back:
This is your bread and butter. Rush it.


Third/Fourth and Beyond Trips back:
Lich bane is going to give you really big damage after using your E and will be great for last hitting champions as they run away. Void staff is going to push past any enemy magic resist and really makes you can absolutely huge threat to ANYONE.


Possible Items for Longer Games
With this items active you'll be able to burst down any champion and also reduce cooldowns while regening some mana.
If the enemy team has a lot of AD Damage, this is great.
If you find yourself taking a lot of magic damage although I'm not a fan of this item.

Gragas is great at farming throughout the entire game.

Throw your barrels at the caster minions and last hit the closer minions. If you're finding yourself under a lot of pressure from the enemy mid, just do your best and wait for a gank from your jungler. Gragas can farm really late, really well if you get behind.

In a fight, your job is to hang back and do BIG damage. I always use my ult right in the middle of their team. This does damage to all of them, separates them, and causes mayhem. After that start throwing barrels and using your dash to chase them down.

When Chasing a low enemy:
+ + will get you a lot of kills when people are running away.

Another great trick, is to use Q and drop a barrel at your feet. The ult the enemy into you, and press Q at the same time. This is HUGE damage and if you can do it by a tower, there are very few enemies who could survive.

When you're using this Gragas build, you need to have some beefy team mates who can take some damage for you. You'll easily be the biggest source of damage on your team so you NEED to stay alive. Stay behind your tanks as any AP caster would and just throw barrels like the drunk fat man you are. Target squishies...they are your best friend.

Throw your ultimate when the enemy team groups up. If you can hit all 5, you're team is going to win the fight. Gragas' ult will drop most of their team below half health and then your team can just finish up the kills, or you can do it yourself with your other skills.

Use it to poke, farm minions, and to do huge damage in team fights. It has a great AP ratio and will do 500+ damage in no time. It's also great for leashing blue for your jungler at the start of the game. Throw it at 1:50 and walk to mid.


This is a great skill for sustaining your mana and health in lane. I pick this up at level 3 and keep it running then entire game. It will reduce your damage taken, and will also cause your to put out a lot more damage with your E.


Use this to finish off enemies who are running. It's great when used with + to chase low enemies. Do not use this at the start of a team fight. You're going to be in the middle of the enemy team and you're going to find yourself really dead, really quickly. It can also be used to run through walls and to scoot around the map really really quickly.


This is the big daddy of ultimates. It doesn't get any more powerful than this. Use it to initiate a team fight, to knock a champion into your tower, to save team mates who are being chased, or when you have blue just throw it for fun. You'll get inside the heads of your enemies and they won't be able to predict your crazy play. It's best used when you just throw it in the middle of enemy teams during team fights.

Gragas is also amazing at killing those enemies that like to recall behind their turrets. If they're below 500 health, just use your E to get close and throw your ult. Free kill.

Gragas is a lot of fun, and with this build I'm always the carry, and I always do well.

Good luck!