Hello , gragas is a very unused character that many people do not enjoy the way he is played and in many ways he is an amaqzing character, i have build him in So many ways that i could not understand the way i had to build him. I finally fund hes build with spell vamp and ability power,
WARNING: this build is a for sure a build you must be expirience with gragas, you are not a tank
you are a burster and you will reck!

pro's/
All spells are aoe
nice passice and W to stay in lane
intteerupt ulties or any spells with your own ulti
Great burst
Amazing range
life saving
FREE to cast spell (W)
HE's E can go threw walls
Cons/
Need mana for all spells
he is not tanky but he can take a punch
very hard to 1v2 or plus

i take runes of ability power cause i find it so usefull on gragas early game,
if you want, you can get 5 % Cooldown reduce on gragas with runes but all of them should be ability power
your W is your mana regen and your health regen is your passive, that is all

Thats all the masteries that you need, liek i said you do not need any in defense cause of your life stealing spell vamp, you need the 15 % magic penetration in the offensive talent tree so you dont need to get anymore items for penetration

Utilitie helps your mana and anything else cause without utility you will run out of mana and health quickly

I Would get a dorans ring first If i didnt get any runes for gragas if you have 2 or 3 big runes ,
so if you have runes get a tome with 1 health potion, with this try not to go ahead play safe but when you start gtting your life stealing it will be easier.

HEXTECH REVOLVER right after when you have enough or when you die, if you lane with a vladimir or kayle or any other character that will get a will of the ancients they do stack inbetween friends but they do not stack if you get 2 of them . If your friend has 1 , you will reck inlane
WIll of the ancients after revolver
Then you get another HExtech revolver for the same reason. DO NOT STACK REVOLVERS THEY GOT PATCHED SO YOU CANT STACK ANYMORE
right after get a hexctechgunblade your melee atatcks will heal you without using any mana so it will help alittle plus the active on it to slow your enemies down

Right after get a revolver again this is for your 70 % spell vamp and alot of ability power
this it the time when you start getting alot of kills , STAY WITH YOUR TEAM

BOOTS OF SORCERY
i do not get boots early cause you dont need it for gragas your range is too great and your body slam will get to your target, IF you want get Boots of speed after your hextech then the revolver

SPIRIT VISAGE:
This helps your passive, your W and any regeneration or healing towards you and it gives health this is very helpful on gragas

Finally a Merollo's Evil tome
You'll finnaly need this for cooldown reduce with your spirit visage this will get you to 35 or 40 % , helps with the AP and you will reck

Remeber: You are not a tank but you can save alot of your friend and commerades with your E and your ultimate , use it wisely

CoolDown reduction is very important for gragas

Get you Q, its your main damage, if you would go tank on gragas or max out body slam, your barrel will be ( USELESS) your barrel is the finishing kill and it is a great AOE spell
your W sould always be the same level as your body slam

your ulti is the trick to get your team kills, if your team is fighting and there running use your ulti to bring them back OR PREVENT CHANNELING CHAMPIONS LIKE NUNU AND ETC....

When i use my spells ,My Q and e i mostly focus on hitting champions And minions early game.
you always no matter what even with FULL MANA you need to keep your W up , Your W can get your last hit on a kill and it will help alot on last hitting minions

FARIMING!!!!!!!!!!!!!! You NEED to get creep score no matter what , you need to stay in lane and try not to recall, cause your passive will always bring your health up without noticing
trust me on this its the best way to get gragas to get fed

IF YOU ARE FED MID GAME HELP YOUR FRIENDS GET THERE TURRETS DOWN AND GANK, gragas is amazing in ganking with hes ulti and body slam , use it as your advantage