Gragas Top

General observation about Gragas Top.

He is a safe pick. You'll likely be able to farm and harass your opponent a bit, and have a good chance of getting out of ganks between Body Slam and Flash, but you probably aren't going to win lane hard enough to make a difference in the match without jungler help or a mistakes on the part of your lane opponent. So, you have to be on the lookout for Teleport ganks to pull ahead of your lane opponent

Gragas vs Rumble

tldr; if you play a farm lane, Rumble has the edge is this matchup due to being resourceless and being able to ignore a lot of your poke.

Rumble can ignore the poke damage from Barrel Roll by using Scrap Shield.

If he is spamming Flamespitter he will push the lane toward your tower.

It is somewhat difficult to last hit with Rumble so one strategy would be to concentrate on pushing the lane towards his tower. However to outpush him you are going to expend quite a bit of mana and you'll want some ward coverage. So a Crystalline Flask + Sight Ward would be useful starting items.

The fastest way to push as Gragas is to place a barrel in-between the melee and the caster minions and then Body Slam the melee minions into the barrel's explosion radius.

One interesting thing to note is that While Electro-Harpoon will slow you, it does not decrease the rate or distance which Body Slam moves you. So, if you are in range for a Body Slam you can still hit Rumble with it or get away from Rumble if he follows up with Flamespitter.

As I discuss above, Rumble can ignore a lot of your harass so to maximize your harass against Rumble you need to do some prep work.

1) Thin out the minion wave to make sure you don't take too much creep aggro
2) Wait until Flamespitter is on cool down.
3) Take a swig from your cask (W)
4) Throw a barrel under his feet (Q)
5) Body slam into him (make sure there are no minions between you and him, and the hit box is rather sensitive on this)
6) Let him enjoy and W empowered autoattack
7) Walk away as flamespitter is going to be back up by this point.

I am still a hack at using Explosive Cask. Unless it lands on someone and kills them instantly I am going to manage to blow them away from me or my team 90% of the time. Need to watch some vids on how to use it.

Gragas as a Support

Works surprisingly well. You can harass with Barrel Roll (try not to push the lane too much and/or take too many creeps from your ADC).

You can peel with Body Slam and Explosive Cask and you have some health sustain in the form of Happy Hour and have some damage reduction in the form of Drunken Rage.

He can burn through quite a bit of mana so you'll want the mastery that makes your health potions also return some mana. I can see making a case for starting with either Spellthief's Edge or Relic Shield, however Relic Shield makes you tankier in the form of additional health and it does offer some health sustain for your ADC that Gragas otherwise lacks making it the preferred choice.