Gragas is a very unique champion that for a long time people just couldn't seem to figure out. He seems to have abilities that are more utility-based and his ultimate gives him ridiculous counter-initiation potential. Tank/Bruiser seemed like a strong role for him. What people came to learn however is that on top of his utility his spells scale extremely well with both levels and AP. Thus AP Mid became a natural fit for him.

Fed up with the slow speed of Fiddlesticks in the new jungle and looking for another strong AP champ to play in it, I came upon Gragas. I'm used to playing him mid and I've always been amazed at how quickly and easily he can annihilate wraith camps as well as blue buff camps-- even with minimal items. This led me to consider him as a full-on jungler, and this guide is the result.

Other than for the sheer confusion of the enemy team in loading screen, there are in fact redeeming qualities to AP jungle Gragas.

Pros:
-High AOE damage leads to fast clear times throughout the game
-Able to sustain in jungle after first clear thanks to passive combined with W
-High mobility and strong positioning in jungle
-Is a versatile ganker and has strong counter-gank potential with ult
-Is a bursty caster as opposed to a bruiser which is useful in certain team comps
-Can hold/push lanes extremely effectively when necessary

Cons:
-Reliant on blue buff start
-Can end up low on health after first clear
-Does not have as much dragon threat as wriggles-based junglers
-Squishy without W buff, especially mid/late game
-Ults can go horribly, horribly, wrong

For runes I mainly stick with what I used as an AP mid, since Gragas' scaling is so crazy and he doesn't have much problem clearing with autos even when he no mana for abilities.

Magic Pen Reds
Armor Yellows
Flat AP Blues
Flat AP Quints

The most necessary part of these runes is the Armor seals. The rest can be experimented with but I like to have strong early game damage so I can secure kills.

My masteries are mainly all out AP offensive with the exception of the 3 points in Havoc because I don't think it's worth it. I'd rather have an extra 4 damage to jungle creeps to speed up my initial clear.

I also go down the defensive tree for the extra armor, reduced damage from creeps, as well as bladed armor. Though Gragas Q slows down creep attacks, 6 damage per hit to them still adds up fast, especially early.

With these masteries it is usually possible to gank after clearing depending on how strong your leash/wolf assistance was. I go all out damage for the most part because Gragas' strength is damage, not tanking.

To start I go Boots of Speed + 3 pots which will help you clear a bit faster as well as gank more effectively early. I've heard of Amplifying Tome + 1 pot working as well but I don't really see much benefit aside from an early Kage's Lucky Pick. If you're really afraid of being invaded early and still want to gank Cloth Armor + 5 pots works-- just don't build anything out of it early on or you'll find yourself falling behind.

Gragas can be equally effective with 2 build routes:

1: Catalyst > Rod of Ages > Rabadon's Deathcap > Void Staff
2: Stacked Doran's Rings > Rabadon's Deathcap > Void Staff

Both of these will alleviate Gragas' early mana issues as well as provide a boost in Health and AP. Which route you choose depends on what your team needs and how well you are doing. If you are fed (enemy team derped and gave you early kills/assists) invest in an early Kage's Lucky Pick-- it will make you gold all game and build into a Deathfire Grasp later.

If you finish all these items then you are either super fed or are super late in the game. At this point look at the enemy team's most threatening members damage-wise and try to pick items that will either kill them (Deathfire Grasp) or keep you alive (Banshees Veil, Zhonyas Hourglass) depending on how team fights are playing out. If you got this far you must have some game sense so put it to use and build toward items that will win you the game. Also don't forget about the pot buffs-- late game they are extremely cost effective.

At level 1 you can probably go either Q or W and finish wolves > blue buff equally quickly as long as you have assistance from your teammates. You'll be taking roughly the same amount of damage either way since Q lowers the attack speed of jungle creeps. At 2 get a point in whatever you didn't get a point in first (Q if you went W, and vice versa). Grab Body Slam at 3 and then your basic pattern is:

R > Q > W > E

R because it's your ult, duh.

Max Q first because it is your primary nuke and it has extremely strong scaling both damage-wise and debuff-wise, getting to 285 AoE damage as well as a 40% AoE attack speed debuff for 3 seconds at rank 5. That means you'll be dealing huge damage to grouped camps like wraiths and wolves, and they'll be doing 40% less damage to you. With your passive giving you health regen near constantly you shouldn't have any health problems after maxing this.

Max W second because once you max Q you will need a good way to restore mana. At rank 5 this ability gives +70 AD as well as 18% reduced damage in general, plus it restores 90 mana. If you find yourself running out of mana a lot before you get your mana regen items, it can be useful to rank W over Q once to get a bit more mileage out of the mana restoration, as well as making you a bigger threat without much mana.

I max E last despite the loss of damage output early on. Why?

1. You'll be spamming this ability all game, a lot of the time just to get around. There is no decrease in cooldown or increase in utility (the slow) so there is no reason to level it in this regard.
2. The damage is spread out amongst targets hit. Basically, unless you catch someone alone, this ability's damage will be spread out in ways you don't want, making it a less than ideal nuke.
3. A lot of times it won't be safe to use this ability for damage in a teamfight or you'll get focused and often killed. It's much more effective to poke with Q and save E for positioning yourself or jumping in when you have a noticeable advantage (long after your tank/bruiser initiates and enemy CC has been blown).

With all that said, there are situations where I would level E over W, like if I get my catalyst/RoA early and I'm not having many mana issues. Still this is somewhat risky as the extra 20 mana per cast adds up very quickly even over the course of a short skirmish.

Smite because it's necessary for jungling and securing objectives.

Flash because, well... it's Flash. I honestly couldn't think of a better spell to use here. I suppose Exhaust or Ignite can improve your early ganks, and Gragas is usually ok positioning-wise later thanks to Body Slam. I still like Flash because it allows me to play aggressively in situations I couldn't afford to otherwise (initiating potentially dangerous ganks with Body Slam, escaping with Flash if necessary)

The way you approach jungle camps will determine how quickly you clear them. Early on ideally you want to throw in a Q from max range, hit W to restore some of your lost mana and apply buff, (if you are low on hp try to delay the W a bit to restart Gragas' inherent passive health regen) then walk up to the creeps and simultaneously auto attack and explode barrel to damage and apply the debuff. By casting Q as early as possible you start the cooldown which ticks down whether you explode the barrel instantly or not. By doing this you can Q twice in an extremely short time and nuke down camps quickly while taking minimal damage.

Later on you can add E to this pattern in order to clear even faster and cut down on the time you'd spend walking up to creeps. The wraiths are a good example, you can throw a barrel from long range, hit W, and then jump the wall in several places to clear pretty much instantly.

As for the route, the standard Wolves > Blue > Wraiths > Golems > Red > Wraiths will put you at level 4 with double buffs around the 3 and a half minute mark. Depending on how well your teammates assisted and how well you timed abilities you will have roughly half health as well. From here you can gank top, mid, or wait for wraiths and wolves to respawn and clear them again. Gragas can pretty much endlessly clear the small camps, but Double Golems will cost you too much health and mana if you clear them repeatedly.

If you know you want to do a strong gank after your first clear you can avoid the double golem camp and instead go back to wolves after wraiths. You'll take less damage this way but it will be slower since you're backtracking.

After your initial clear use your own judgment. If the enemy jungler shows themself top and you can't get there in time, head into his bottom jungle to steal creeps, if an enemy laner is being overly aggressive, camp the lane and punish him. If there is a strong early ganker in the game like Shaco, Maokai, Lee Sin, etc. try to keep track of them and assist the lanes they attempt to gank. There's countless other things you can do but this isn't a guide on general jungling.

The best way to gank with Gragas is to have your teammate(s) start a skirmish with the opposing laner(s) and Body Slam on them when they have committed to a fight. From there they have two choices:
1. Blow summoners in order to escape (in this situation sometimes a kill can still be secured if they are low or you have your ultimate)
2. Go all in and try to kill you or your teammate
If they go route 1, you have successfully pressured the lane and most likely damaged them as well. They will start to play more passively and possibly be zoned from Gold and EXP. If they don't start playing passively, punish them by ganking again afterwards and they will not be able to escape.
If they go route 2, that's the sign for you to nuke them with everything you have, including your ult, if possible. Whether they kill your teammate or not (hopefully not), you will come out ahead with a kill and an assist to their single kill.
In either case be aware of the potential of the enemy jungler to intercept and counter gank.

Gragas is one of the few junglers that can successfully gank a warded lane without resorting to a situational lane gank through brush (though those can be very effective). If an enemy lane is relying on a ward near Dragon or Baron to protect them, Gragas can easily slip past it by jumping the cliff wall. Even when he is spotted, often you'll be able to cut off an overextended enemy by repeatedly Body Slamming, somewhat like a Rammus Powerball.

Lastly, be very careful and precise when using your ultimate. It will determine whether your teammates rage at you or praise you for your awesomeness.

This is somewhat obvious to most Gragas players, but your role in a teamfight is two-fold.

Before the team fight erupts/jockeying for position:
Poke, poke, poke. Repeatedly Q into the enemy team, ideally the squishy AD or AP carries, and chunk down their health. With good amount of AP (300+), you'll be able to chunk squishies for 1/2 of their health with just 1-2 Qs. Of course, they won't stand still, so be persistent.

Once the teamfight starts/someone initiates:
If the fight is in your favor, i.e. you successfully chunked their squishies before the fight began, throw out your Q and possibly your ult if you see an opportunity to insta-gib someone. Get in position to use Body Slam to intercept weakened targets trying to escape. If a strong enemy bruiser or assassin dives you or another friendly carry, don't hesitate to ult them away if you haven't used it yet.

If the teamfight is even or you weren't able to give yourself much of a lead beforehand, ult into the middle of their team (or in the middle of as many of them as you can) to separate them as much as possible so your team focus a single target quickly and gain an advantage. A lot of times after this the opposing team will be in disarray, especially if your ult knocked one or more of them over terrain. If they regroup and fight try to make yourself seem accessible while spamming Q. If enemies try to focus you, reposition yourself with E. Once the fight starts going your way, use E to slow vulnerable targets. If your team is losing, save E to make a quick exit and spam Q in the enemy's path if they chase.

Please note that this is just a general description of how you want to perform in a team fight. There are an uncountable number of different team fight situations, which is part of the reason why LoL and other DotA type games have so much replay value. Above all, personal judgment and game sense are much more important than following the advice of a guide.

I didn't make this guide to say that AP jungle Gragas is insanely strong or even just viable at high levels of play. I made it to show that it is a possible, and surprisingly effective way to play this champion.