Gragas is a very solid pick in today's meta game, with any team composition. Gragas is very flexible: he can lane, he can jungle, he can build tanky, he can build like an AP carry, he can even build AD. In this guide I present 2 builds for Gragas, both of which are focused on laning: AP Carry and AP Bruiser. With that said, I also present a jungling variant, though I don't recommend it because: Gragas is such a beast in lane, his ganks aren't very powerful and he can't get an early(levels 4-6) dragon by himself to assert dragon control.
Gragas has been 1 of my favorite champions for quite some time now, and until recently(when Shushei rose to popularity and became known for his Gragas play) very underused and underrated. These days he's seeing more play than he used to, thanks to Shushei's positive influence. However I feel that he's still underused and that there's a of potential in him that has yet to be tapped and exploited.
The first build has pretty standard caster runes. I chose AP/level because Gragas is already a beast early game, that I feel the additional AP early on isn't as necessary as with some other casters. Also, going for Rod of Ages first makes it so that the per level glyphs catch up before you complete the Rabadon's Deathcap, so when you do you get an even larger bonus AP. For Quints I still go flats because the scaling ones catch up too slowly and the additional AP early on will make sure you dominate your lane. You can of course tweak these runes to your liking, like going for defensive seals instead of mana regeneration, health/MS Quints instead of AP, etc.
The second build has rather none standard runes, so let me explain them a bit more. I chose AD/level marks because your E stacks with both AD and AP and because it stacks nicely with W's additional AD. We don't need flats because with the bruiser build we're more about sustaining ourselves than trying to kill our lane opponent early(considering that he'll most likely be a bruiser as well, so it'll be nearly impossible to do so to begin with). Also the per/level marks catch up quite fast with the flats, so it's easier to take the scaling version. Moreover, since it's your only source of additional AD, might as well make the most of it. I chose dodge seals to go along with the defensive masteries and the odd match in which you build Ninja Tabi rather than Mercury Treads because dodge stacks nicely with your damage reduction from Tenacity+W, to give you even more defense that isn't armor reliant, thus can't be reduced by penetration. Glyphs are pretty standard for bruisers, with MR/level. As for Quints, I chose MS to give you that little extra speed(especially early on in the lane) to get in and out with minimal harassment. However you can choose any other Quints you prefer, Fortitude being a very popular and common alternative, but others can also work. You can of course tweak these runes to your liking, like ditching the AD marks for something more standard, but I find that this somewhat specialized rune set for the second build really works well.
First build has very standard 9/0/21 masteries, don't think I need any further explanations. The second build is somewhat unorthodox with 9/21/0, and especially with the very underused Ardor. The reason I go with 9/21/0 for the second build is because the 15% magic penetration is still godly, considering that all your skills deal magic damage, and unlike the first build, as an AP Bruiser I find the defensive masteries bring a lot more than the utility ones. Other Bruisers can get away with not running defensive masteries as they get mostly pure tank items like Randuin's, Atmog's, etc. As an AP Bruiser still needs some AP to be effective, so your defensive items are not as powerful(as far as defense goes, obviously they offer much better offense) as those most other Bruisers use, therefore defensive masteries are almost a must have. As for Ardor, I find that AP Bruiser is perhaps the only role that really benefits from this very underused mastery, and Gragas is in the perfect position to utilize it. More AP later on is precisely what the doctor ordered. The additional AS, though minor, is still a nifty little bonus, and stacks well with Wit's End. As a Bruiser, despite focusing on AP rather than AS/AD, you'll still be auto attacking, and with your runes(scaling AD marks) and W, you'll have respectable AD even with no AD items at all.
Both skill sequences are pretty standard for their respective roles. The only reason I also start with Q on the AP Bruiser build is because it gives you the most bang for your buck at level 1, and helps you assert lane dominance or to catch up on your farm in case you were away from lane early on(protecting your jungler, level 1 team fight, etc). Q is also the best choice for a level 1 team fight for both builds, which gives you even more of an incentive to start with it for both builds.
Ahhh, the item section, the bane of every guide out there. Before I start, I have a disclaimer. Item builds are not set in stone! The most important thing to know about item builds is that they're ever changing according to the circumstances. With that said, no guide is complete without an item build, as that's the core of most guides. Thus I've recommended 2 sets of items above, for the 2 different builds in this guide: AP Carry and AP Bruiser. These builds should serve you well against most standard(balanced) teams when the match is flowing smoothly. If you find yourself against non balanced teams(stacked towards either physical or magical damage, overly squishy, overly beefy, etc), are snowballing, are letting someone on the enemy team to snowball, etc, you'll obviously need to make adjustments.
Personally I dislike starting with the Doran's items, and try to refrain from doing so whenever it doesn't hurt too much to start with something else. Gragas is a perfect example of a champion that really doesn't need a Doran's item to dominate his lane early on. Therefore I start both builds with a Sapphire Crystal and 2 health potions, building 1 into Catalyst the Protector and the other into Sheen, both of which are terrific first medium items for the laning phase. From this point the builds diverge.
The first build continues on towards Rod of Ages for the raw stats(especially AP and hp), followed by Rabadon's Deathcap(no AP Carry is complete without it). Afterwards I go for Banshee's Veil as my main MR item, though you can certainly pick Force of Nature or especially Quicksilver Sash instead, according to your preference and the circumstances(for example Quicksilver Sash is much superior to Banshee's Veil against suppression ultimates(Warwick, Malzahar, etc). I than continue for a very standard Void Staff, though if the enemy team is not stacking MR for some reason than there's obviously no incentive to build it, choose something else instead like Rylai's Crystal Scepter, Deathfire Grasp, etc. Lastly I build Zhonya's Hourglass for more AP, and more importantly armor and the active component, as it can be the difference between a won and a lost team fight, which can be very crucial late game. Again though, if the circumstances dictate different items than by all means choose whatever fits best.
The second build continues towards Spirit Visage which stacks very well with your passive, and gives some nice cheap MR and CDR. You can choose to skip this stage and get a different MR item instead of you want. Afterwards I usually get a Glacial Shroud for the armor and CDR, waiting a bit before completing the Frozen Heart unless the enemy's AD Carry/Bruiser got fed and snowballed early, in which case I rush it. You can choose a different armor item, like a Thornmail or Sunfire Cape if you want, though in most cases I find that the Glacial Shroud/ Frozen Heart is the better item thanks to the CDR(nearly maxed with it and the Spirit Visage and the AS debuff. The next item is Abyssal Scepter, MR, AP and MPen, the 3 most important stats for an AP Bruiser, which make it the perfect item. You can pick a different item but I strongly advise against it, as it's just too perfect. At this point you have enough AP to make it worthwhile to finish the Sheen into a Lich Bane, so go ahead. I finish with Wit's End because the additional AS and damage on hit helps to keep us relevant into the late game, when most tanks and bruisers start to lose their effectiveness. The extra MR is also very helpful of course, and as a bruiser it will stack pretty fast when you get onto your target and auto attack it. I especially like Wit's End because a smart enemy team will not focus on you being an AP Bruiser but rather use up their ultimates and big damage spells on your team's squishies, therefore the spells you will take will more often than not be the second wave(spells that just got off cooldown), and by the time those will come you should have maxed Wit's End's MR bonus. However, again, if you feel a different item is better in its place than by all means use it. A few examples are Rylai's Crystal Scepter, Zhonya's Hourglass, Force of Nature, etc.
Farming has never been easier with the combo of Q+E, and eventually just Q(especially for the AP Carry build). Last hitting in lane is also a breeze when you got W on, and you should have it on at all times anyway.
Creeping / Jungling
As I said in the introduction, I'm including a jungling section though I don't personally recommend it. If you insist on going Jungle Gragas than I'll definitely recommend the second build, tweaking your masteries to take Improved Smite, and reducing points from the defensive tree for at least 10 in utility(experience and buff duration are a must for any jungler). You'll also need to level your skills a bit differently, WEWQ is how you should be starting, than the same R>W>E>Q. As for the jungling path, Gragas is very flexible. He isn't reliant on blue so you can start anywhere, Twin Golems and Wraiths are good places to start if you don't want to start at blue. you can also start at the enemy's Wraiths if you want, considering that most junglers either start at blue or Twin Golems, stealing the enemy jungler's Wraiths early on can really mess him up and is relatively safe to do.
Pros / Cons
+ Naturally beefy, especially compared to other AP Carries.
+ Explosive Cask is a terrific initiation as well as counter initiation skill.
+ Amazing lane pressence.
+ Body Slam makes for some amazing chases/escapes.
+ Very flexible, and therefore hard to counter pick since you can use 2 opposite builds.
- Skill shot based(Q+R).
- AP Carry build only uses 2 skills for damage(E is generally too risky for offense).
- A big target, ok for the Bruiser, bad for the AP Carry build.
Gragas is a very flexible champion, he is naturally beefy thanks to his base stats and Drunken Rage, therefore he can still be built squishy and enjoy being tougher than your standard AP Carry, or tanky and have more damage than your average Bruiser. He can lane, he can jungle(though not recommended), and with the various builds available to him(to which I only listed 2), he is very hard to predict during the draft phase, and therefore very tough to counter pick. Overall Gragas is a great pick for any team composition in today's meta game, and he can fill 2 main roles: AP Carry or Bruiser(either AP like I recommended in this guide or AD which I haven't touched upon).