Gragas is in a pretty good spot right now, I think he's a great toplaner and jungler, I've even pulled off support quite a few times.
You're maxing RQWE right?
I would avoid building the Elder Wraith on him, just go Golem into RoA. RoA is all the damage he needs, he deals quite a lot of damage over sustained fights, while also having the QWE burst combo too
Golem>RoA>Spirit Visage or Sunfire (randuins if they really have a lot of escapes, otherwise you can stick to them and deal the fire dps), just build resistances based on their teams damage outputs. I'd avoid Chilly Mittens and morellonomicon on him, Gragas just wants health for his passive so he can deal his steady dps and disruption as long as he can.
I roll my AP jungler page (5 AP, 3% MS, dual pen reds, AR yellows and AR/MR blues) with 9/21 masters, always grab that double edged sword for grag.