I am a jungler with my friends, i try jungling strange champs on occasion and i stumbled upon jungle gragas. the benefit to having a gragas in the jungle is ganks with his damage and ultimate are really strong and you can choose to build more tanky if your team needs it. or else you can do insane burst damage. i wasnt sure how his early jungling would be but with proper runes and masteries it is very very managable and you can also try a 9/21 mastery build but i have not tried it out yet as i prefer a burstier gragas.

Pros:
Great Ganks
Brings great CC to the team and can initiate well
pretty fast jungle speed

Cons:
needs a good hard leash on blue to make jungling easier

Wolves>Blue>Wraith>Wolves>Gank Mid/Top>Wraiths/Wolves

starting wolves lets you hit level 3 on your second wolves kill which makes your mid or top gank really effective. you can at least push the enemy mid out of line with a nice charge/slow in and the damage from barrel toss.

The build above goes for a more bursty Gragas but can still tank a good amount of damage because of Gragas' high health. Getting the Double GP10 is important to me because of the philo stones regen. sometimes depending on how much gold i get from ganks ill skip the heart of gold as i don't build it into anything later on anyways but it is still a good item. Rod of Ages is such a good item on Gragas i cant see you ever playing him without going for one it gives him the AP and so much health for the early-midgame. and i like Building Shurelya's even though the support might have one giving it to Gragas makes his chasing and running away really strong with his Charge Spam.

Gragas is a fantastic ganker his ult has a short cooldown which makes him able to roam around and have successful ganks frequently. i take ghost because i like being able to utilize his charge/slow and his high health which lets him be able to turret dive and get kills at an earlier level than most champions.