Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Want to lane 1v2 and still get first blood? Want to be an amazing last hitter? Want to go wherever you want on the map, at any time? Then Nuclear Sub is for you. Make sure you lock in the win by having a teammate go to the jungle. 2v2 Gragas is 3rd teir. 1v2 Gragas is God Mode. Move stealthily through the water in your scuba gear, ready to unleash nukes on some unsuspecting North Korean peasants... I mean enemies. You are only a nuclear submarine if you have the Scuba Gragas skin, though. No imposters! The following guide is written in both Fat Tank! and Nuclear Submarine context.
You need magic penetration to amplify your damage since you won't be getting any additional spell power until late in the game, if at all. Dodge will give you extra survivability as well as a 10% move speed buff in almost all combat situations. Lastly, Gragas needs mana regen per level to spam spells to keep his health up, get last hits, harass and kill enemies, and be an all around unstoppable force. I add move speed quints to get him over 400 movement with level 2 boots, which helps with map presence and positioning. Lastly, I threw in a 2% extra xp quint to really add an overleveling component when combined with the masteries. Gragas has the luxury of being able to pick these quints because he is already survivable early game and does not need HP or penetration in those slots.
If you don't have these runes, buy a level 1 set until you can upgrade. They're 60% of the benefit for 3% of the cost of level 3 runes.
15% spell penetration is miles behind 10% move speed after dodging for a tank/offtank, and the rest of the utility tree far outshines the other trees deep talents. These are really a no-brainer.
Fat Tank needs fat, so start with a Doran's Shield for early staying power. Next, get an Amplifying Tome early to truly wreck minion waves and the opposing champ(s). After that, Warmog's Armor is easily farmable. I get it immediately after level 1 boots, if not before; it depends on how well my farming and killing are going. This item will double your health pool, which means it doubles the effectiveness of your passive and the armor and magic resist you are about to toss on top. Force of Nature is my favorite next item, after getting level 2 boots of course, because of the sick regen and movement speed, but Banshee's Veil; Warden's Mail/Randuin's Omen; Glacial Shroud/Frozen Heart; and Thornmail are all excellent next items, depending on the other team. Get as many of these as you like. Once you've got a solid combination of health, armor, and magic resist, you can boost your damage by beginning to build AP items like Lich Bane (Mage Masher lol) and Rabadon's Deathcap, starting with a Sheen.
As I'm sure you've noticed by now, this build varies from other builds because it does not focus on mana or AP items. Instead, I rely on Gragas's superior positioning gained by Body Slam and Explosive Cask to create kill opportunities for myself and teammates. Tank items maximize these positioning skills by allowing you to be a wall of protection for your team after you isolate one. This is also why Clarity becomes particularly important for this build.
If at any point you feel the game is ready to end, you can spend 900 on the three buff elixirs, load up the nuclear missile tubes, and go to town on some unsuspecting Middle Easterners with oil. Does irradiated oil still burn well in cars? =/
Explosive Cask > Barrel Roll > Drunken Rage > Body Slam
Body Slam has less overall damage than Barrel Roll, but provides enormous utility in all stages of the game. One point at either 3 or 4 is absolutely necessary. It has a very long range which is only used if you target well past the end of the targeting reticule. You must take advantage of this fact if you don't want to faceplant into walls and probably die to ganks because of it. You can use this ability for travel, hopping over almost any wall, positioning yourself away from a gank, escaping turrets, putting yourself in a good place to toss your ultimate, and of course the 35% slow. Sheen amplifies the damage you do after this ability wonderfully..
Drunken Rage will allow you to stay away from your fountain until you find yourself with lots of gold and no mana. At this point in time, you may allow yourself a refreshing drink and become the tank you were meant to be. DO NOT MAKE FREQUENT BUYING TRIPS. Take all the XP and champion kills you can find. If you run out of mana, have no clarity, and can't get a blue buff, take one of your few trips back to the fountain.
Barrel Roll is a long range, area of effect nuke that slows the attacks of those hit. Harass often, and try to land as many creep kills as possible. I like to throw a barrel down with as many minions in range as possible, then damage all creeps so that I get the whole wave at once when the barrel explodes. It's your basic frag grenade, that you control how quick it explodes after the pin is pulled. Pretty sweet, huh?
Explosive Cask is Barrel Roll's big brother. It has the same monstrous range, it does more damage, flies faster, and repositions entire teams. I use this to knock opponents into my turrets, my body slams, my team, over Anivia walls, away from a swarmed teammate, away from me when they come for a gank, and even as a simple finishing blow. Don't forget to Ignite before you toss the nuclear warhead cruise missile on top. Even if they don't die immediately, Ignite radiation will probably still get them and the next generation to come.
Clarity will allow you to use your spells all day which will trigger your passive health regen, as well as fuel unanticipated damage if your opponent attempts a gank. Clarity will also allow your team to push for the win instead of a trip back to the fountain after a big team fight. Ignite will get you all the kills you deserve so much. Gragas does not need flash or ghost for positioning because of body slam. He does, however, need a way to finish his kills and obtain the rewards. He won't need teleport because of Clarity, his passive, and Drunken Rage. Honestly, I can't think of anything even remotely as good as Clarity and Ignite on Gragas.
Creeping / Jungling
After you get your Giant's Belt, you'll easily be able to take the buffs. You can alternate pushing your lane with going to the jungle to get buffs on the top side. Once you have them, waves and champions have little they can do to stop the onslaught. Get them whenever possible, especially late game. If they have a jungler or are missing, encourage your teammates to come with you. You'll probably end up with kills and buffs.
Two level 5 Barrel Rolls will eat a wave at any point in the game. After you get your Amplifying Tome, you can wipe the ranged minions by following up with a Body Slam. Try to take as much experience off the map as possible. If you have time to run to the jungle to kill minions and come back and not miss any wave experience, do it.
Buy and place wards in forward positions with your leftover money after every buying session. If you are buying too frequently, you may end up buying too many wards. Also be prepared to buy an Oracle Elixir if they have even the faintest bit of invisibility. Otherwise, stick to the plan, chums.
Low Level Play
If you decide to main Gragas, spend the IP to upgrade your runes, focusing on mana regen. You should spend your first 11 points getting Meditation, then get increased buff duration, movement speed, and improved Clarity in whatever order you prefer. Once you get enough point to spare 9 in dodge, switch your yellow runes over to dodge.
At level 6, and particularly if you get there before your opponent(s), you are at a great spot to get some easy kills. Whittle the enemy down with constant Barrel Rolls, then kick the real party off by using Explosive Cask to knock your opponent behind you and immediately Body Slam him. Ignite, Barrel Roll, and one last Body Slam is as close to a guaranteed kill as you can get in LoL.