Gragas as a support is great against other tank supports, and works pretty well against enchanters/mages. His CC and kill potential often let the allied carry snowball and win the match. He is a very versatile pick, works great as a tanky protector with disengage, and as an engage tool. You can blind pick him anytime and find success in the bot lane. Personally, I ban Lulu in every match. She's probably the strongest enchanter and works way too good with certain AD Carries, making them almost unbeatable 2vs5 combo. (Twitch, Vayne, Kog, even Ashe). I advise you to ban this champion as well. It's the best thing to ban at the moment.

The laning phase is pretty easy, but you'll have to practise for a while before you manage to perfect everything. Your passive healing helps you in keeping your health bar full. The base damage from E is also good, even after the damage nerf. In most matches you'll want to quick trade and back off or just go in and kill your enemies. Your powerspikes are obviously level 6 and level 9, when you unlock your ultimate, and have your E maxed out. After 6, you can destroy any bot lane or make them lose their summoner spells with your ultimate. Don't be scared to help your jungler in killing crab, Gragas's early combo does some nice damage and it will scare most of the people. Remember to often play around your carry, for only you are able to save them with your reliable CC.

You can check my win rate on op.gg - Graggy is a legit pick. If you want to win with him, you'll have to learn some basic tricks which are probably covered in other Gragas guides, but I'll still put them here.
1. W+E animation cancel
Whenever you're going for a trade with charged W, and the enemy support is nearby, you can auto attack them, and use E during the W attack's animation. It will cancel the remaining W animation and let you deal more damage in less time, letting you back off faster and preventing from losing trades.
2. E + charged W + flash combo
This is the most popular and used combo, simply because it's pretty broken. You'll be able to catch most of the players with it. You charge your W, use your E in the enemy's direction, and flash right before animation ends. If you flash too early, you will ruin the combo and not hit anyone. Try it out in practice tool.
3. E + charged W + flash + R combo
This one is the most spectacular, and probably the most useful combo. You basically do the same thing that you do in number 2, but after hitting your enemy, you throw your cask behind them so they fly right in your team's range. You'll want to use this combo on fed players and all the enemy carries. If executed well, this is a certain death for them. Works best against a fed ADC, if they dare to walk up close enough, they will drop dead.
4. The unexpected bomb (W+R+E+Q)
I only use this one during the laning phase, and if the enemy carry oversteps. Ping your ADC before you use it to make sure they will have enough time to respond. You charge W first then throw your cask behind the enemy, and as they arrive in the destination, you use your E for the chain CC, hit them with W and Q for extra damage. If they don't flash during your ult animation, they won't survive.