IMO Gragas has very powerful early ganks. You'll want to use this on lanes that need it to most (mainly depends on your comp) typically the top lane is the easiest one to secure a kill in the beginning. Combining your flash with E can be very surprising and startling for the enemy which almost always results in their flash, this is when you would use your Q and hope your top laner follows up. If your top is pushing lane and isn't giving you an opening then you can either proceed right into enemy jungle or bot lane. If mid looks like it can use the help, then offer it.


Now, there isn't much I can explain to you. Like I said I expect you to already have some knowledge in jungling/carrying. All I can say is to don't be afraid to get aggressive, if you time your W correctly you'll win the fights. You can save E to always ensure a getaway or use it for a quick stun, but be careful of enemy laners helping.


You always want to be smart with your time. If enemy jung is ganking top, try to set something down bot to snag an easy drag. If you're in the other position, hope you're close enough to steal! And enemy team should never get baron if you play it right. The damage with Q+R on top of smite (if it's needed) should be more than enough to guarantee steals.


If your team is losing you have to look for easy targets. Always let your Q ferment before the wave comes in and pretty much everything else comes down to how you use your ult in team fights. If you built more damage than defense, you can look to one-shot their squishy targets (given if they don't have any one like Lulu that can mitigate the D.)