Guide has been created! It is severely lacking due to my lack of sleep and possible brain damage. I will continue to add more as it occurs to me.
Heres results from today's Jungle AP Gragas games. (only had time for 2 jungle gragas games today)
9x Magic Penetration...
Magic pen for ganking/teamfights.
Armor for jungling.
AP for more damage.
Nothing too mind-blowing here folks.
9/11/10 Masteries have worked best for me so far. This is also where this guide might be criticized the most, for some reason many people hate spreading out their mastery points.
Offense tree gives the AP and 10% magic penetration for fighting champions
Defense tree gives the defenses (go figure) to jungle as well as a small bit of health
Utility tree gives you the movement speed for ganking, and the buff duration increase
I found going fully into any tree ended up with inconsistent results, my goal was to have a build that would work whether you got a kill/assist from early ganks or not, and leaving the jungle gank ready.
Cloth + 5 Most standard jungling setup, builds into Guardian Angel later on
Sorcerer Shoes adds a good amount of damage to your ganks, if the enemy team has a strong amount of in your face cc, go for merc treads. Thanks to Gragas almost always having perfect positioning thanks to his Body Slam, I rarely buy merc treads, but merely avoid the cc in most cases.
Rod of Ages Great item on Gragas! Great sustain through the leveling up passive, and strong stats all around, get early to max stacks to be ready for teamfights
Deathfire Grasp DON'T FORGET TO USE THE ACTIVE!!! Seriously, if you forget to use the active don't bother buying this item! I personally like to place it in the 2nd inventory slot because 2 is easier to press in a pinch than 1 for me. This item is what will intimidate the enemy assassin/carry and keep them far away from you. In most cases, a quick DFG active, R,E,Q (and ignite if needed) combo will flat out kill someone. This is ALWAYS my staple item for gragas, it just compliments his kit so much. The cooldown reduction is nice too! (naturally, buy the gp5 item that builds into it first to discount the overall price slightly)
Rabadon's Deathcap I cannot think of a reason to NOT build this baby, its the casters version of Infinity Edge, a.k.a. Must have!
Void Staff For when those pesky bad guys catch on to your damage potential and build Magic Resist. Combine this with Magic Pen. runes and 10% from masteries, you'll easily be cutting through 50% of their resist. Great for handling bruisers/tanks when no other target is available.
Guardian Angel Because by the time you can afford this item, the enemy team will have realized how much of a threat you are and will start focusing you. Gives great defenses and an extra life.
This is usually my straight and narrow build. However, sometimes I will buld Doran's Rings (2 at the max) if I need a small boost but don't have enough gold to buy the next items needed.
Creeping / Jungling
Now this is where you have a choice, you can take your charge and gank, or you can put another point into your barrel and continue to clear.
Honestly this is probably the most standard jungling route so once again nothing too mind-blowing, and I won't try to insult your intelligence by telling you every minor detail. (After all you are smart enough to be playing Gragas!)
Something that many people do not realize: Q's cooldown starts when the barrel is thrown, and you have about 4-5 seconds before it explodes by itself. So if you have a competent leasher who can do it asap, you can throw your barrel on blue golem a few seconds before it spawns to cut your next Q's cooldown time in half. I also use this technique by throwing a barrel before I drink (W) when engaging the next camp. small tricks like this will help quicken your clear time.
With blue buff you have enough mana regen to use your Body Slam to scoot across the map faster to further speed your clearing.
Invading: While Gragas isn't the best 1v1 champion around, he can usually invade while staying relatively safe thanks to Body Slam. Simply slide over a wall and chuckle.
Thanks to Gragas' nuking power on his Q, clearing gets to be insanely fast when some AP is built, so invading gets easier and easier as time goes on, although it is always best to ward the enemy jungle if you plan on doing it repeatedly or after the lane phase is over.
While everything about Gragas is unique, the skill that is most unique is Explosive Cask, especially when jungling.
Example: An allied lane has harassed an enemy to half hp and that enemy is now sitting at their turret but refuses to recall. Well lets mosey on up to the bush behind their turret (charging over walls can make this faster if they start to recall) Don't forget to ping the target and have your teammates prepare to get the killing blow! Now that you and your teammates are in position, go ahead and toss that R to push them out of the protection of the turret. You can even use Body Slam to get close quickly so they cannot react.
In my experience as a failsafe, I throw Q right where I threw my R immediately after simply because for some weird reason, people run directly back to where they were before the knockback.
Gragas is one of the few junglers who can pull something like this off!