This is my first guide but I have had tons of success with this build on Gragas! I will do my best to flesh this build out completely so you know the do's and don'ts of this champion.

His skills are pretty self explanatory but there are a few tricks to know before you become a Gragas pro. Your W is the key to keeping Gragas alive in fights. It has the lowest cooldown of all his skills plus it reduces incoming damage to Gragas for a short time. When you are ready to gank a lane use your W a few second before you engage in the lane, then use E to just to your target and stun them, and follow it with a Q to slow your target. Early game this might be enough to get a kill by just repeating this process. Once you get your R there are a ton of ways to use it. If an enemy is running from you try to throw it in front of them to knck them back into you and your team. If an ally is in trouble try to use it to knock enemies away from them so they can escape. If you need just a few more seconds to finish dragon and the enemy is coming throw it to knock them back and buy yourself a few seconds. Also do not forget that your E can jump over walls and can save your life if used correctly, and can also turn a 1v1 that your enemy thinks he will win into a 2v1 that they cant handle.

PROS:
Super tanky
Crowd Control for days
Heals from spells every 8 sec.

CONS:
Very low damage late game
Average clear times in jungle
Suffers when team doesn't stay with you.

Start at Gromp then move to blue buff followed by wolves, then finally red buff. At this point you can keep farming or go to a lane and get some kills. Gragas's ability to get successful ganks early in the game is extremely high. Feel from to bully the enemy team while you still have relatively even damage ratios with them early game.