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.
This guide is a work in progress. Until it's finished, and I will declare it finished, I would recommend not commenting. I'm a bit busy, and this guide will be worked on as I can do so.
I've played Gragas for awhile now, and while tanking is great, I found that making him into a DPS was a nice break. He feels a lot like Nasus end-game. I've tried building him as an AP, but it wasn't as effective as I would have liked.
Masteries on Gragas are a bit odd. He isn't a full tree one way or another, since he's so diverse. Any way you build him should be fine, though, but the main focus is on his ability cooldown reduction.
Begin with a Sapphire Crystal. Your mana pool is horrid early game, and you'll be using your barrels quite a bit to keep enemies at bay. Gragas is a great solo with his barrel, so go to town solo, or with another champion. Keep at it until you have enough for a Manamune. I know most are against it, but it helps with your mana pool until it is replaced later in the game.
Boots wise, grab some Berserker's Greaves. The attack speed bonus is nice for how you're going to build him. If you favor Ionian Boots of Lucidity, they're also a nice replacement.
Build a Stinger next for more attack speed. We're aiming for a Nashor's Tooth for the cooldown reduction, attack speed, and ability power.
After Nashor's, start on a Guinsoo's Rageblade for even more attack speed and ability power. During this time in the game, feel free to sell off your Manamune before the Rageblade. You should be pretty well set on mana by the time you're here.
I usually don't get past Nashor's in most games, but if you do, head for a Hextech Gunblade next, starting with a Hextech Revolver. The spell vamp helps keep you alive with your barrels, along with the added ability power. I cannot stress how much you need that revolver first.
Youmuu's Ghostblade is next on the list. The cooldown reduction and ability of the item are great endgame, as you can literally tear people apart when it's up.
Barrel roll, barrel roll, barrel roll. This ability is your bread and butter as Gragas. Use it to farm, solo, and wreak all sorts of havoc. Get used to throwing it behind enemy champions, because once they see it, they'll usually run. Since you set it behind them, they'll be right on top of it. A great harass all around. Remember though...it's a skill shot.
Always make sure that your Drunken Rage is up. The buff from it is wonderful at all times during the game, and you'll need it for mana early and mid game. Cooldown reduction makes this abilitie's buff stay on, as long as you're using it. Normally, with no cooldown reduction, you will have a brief period of time without the buff.
Body slam should always be used on champions that are too close to a turret, are running away, or in the middle of minions at times. You can also use it to go through thin walls, and it's a wonderful escape tool. With cooldown reduction, you can Body Slam just about every 3 seconds, making it almost impossible to catch you. I've outrun Yi's with their ults on, and that's saying something. Make sure to use Body Slam to gain ground faster as you're running towards your target on the map.
Explosive Cask should be used to bring enemies to you, to keep them away if you're retreating, and to break them up if they're all together in team fights. When you're being chased, Explosive Cask can be used to push them away. Use it when an enemy gets too close, to bring them closer. It's wonderful to single people out in team fights. However, with all skill shots, you can miss. Be careful, and aim carefully.
Flash is used for chasing and fleeing. Gragas is slow normally, and Flash makes up for it. Add body slam to this, and you've got one fast fat man.
Exhaust helps to chase down pesky champions who run faster than you do...and that's a whole lot of them at low levels. I don't find myself using it to run away at all, since Flash and body slam are usually good enough.
I usually only use those two.
Feel free to use Teleport if you feel the need to defend towers. I used to use it until I discovered the glory of flash.
Well, as said, this is my first guide. Feel free to point out any flaws, as I still need to clean this up a little bit. I'm still adjusting this build as I play it, so bear with me.
Thanks for reading.