Gragas’s Ultimate R is very good in team fights. It deals a lot of damage and can displace the whole enemy team.
He has good sustain in lane (compared to many other champions) with his W. This will allow him to look for trades more frequently and heal back after a trade.
His E can be used in conjunction with Flash to get surprise trades/engages off with the enemy. He can also use it to interrupt abilities and enemy dashes.
His Ultimate R can easily be dodged by enemies who have dashes or any movement tools.
If the enemy has good positioning in lane, it will be very difficult for you to engage or use your E in the lane. This is because your E can be blocked by minions if the target is not directly behind them.
Gragas’s Ultimate R is very easy to mess up. You may want to use it aggressively to knock someone forward but instead, knock them backwards.
0 - 15 min
Use the lane brushes to hide and poke the enemy laners with your Q. It should prevent them from trying to all-in your ADC in the lane.
Remember that your E can be interrupted by minions. This is why use your W to get close to them and then E to CC them.
Once you get your Ultimate R, you should have a lot more effectiveness in fighting enemies. Try to separate the enemy ADC from their Support for maximal effectiveness.
15 - 25 min
Make sure that the neutral objective pit is out of bounds for the enemy Jungler during fights. It will help your Jungler secure the objective quickly and without any risk of missing smite.
Always respond faster than the enemy Support when fights break out in the river. Due to your kit, you should have quite an easy time all-in'ing and disrupting escaping enemies.
Roaming is going to help you a lot, especially when your ADC is not in the lane. Use your E, Q, and Ultimate R to completely cut off an enemy laners escape while roaming.
Depending on your build, you will either be dealing tons of damage or tanking a lot of damage for your allies. Whatever be the case, remember that you still need to peel for your ADC.
Try to work with your Jungler and get picks inside the enemy Jungle with your Ultimate R. Even picking off the enemy Support will grant your team a considerable advantage when it comes to vision placements.
Set death brushes when possible and use your Ultimate R to bring unsuspecting enemies close to you. This will let your team collapse on them and kill them.
The lane brushes can let Gragas assert a lot of dominance during the early game. It can allow him to zone enemies out, and he can also use the fog of war to go to some other lane and help his teammates. His Q makes up for some decent poke as well.
level 3 is a massive powerspike for Gragas as he can now look for full-blown all-ins on the enemy target. Positioning may cause some hindrance as his E can be interrupted by minions. Using the lane brushes or his Flash should quickly solve the problem.
Gragas' level 6 is a significant power spike because he can now displace key enemy targets during a gank or all-in. It can let him pick off the enemy backline should he decide to go for the entire AP build. He can also work with his Jungler and pick off enemies with this ability.
Once Gragas manages to get to level 11, he will have two points in his Ultimate R. This means that he can initiate fights and displace key enemy targets way more frequently.
At level 9, Gragas will have his first ability maxed out. This is really beneficial for him as the damage increase will really help him duel. This is especially true if Gragas is going for the full AP build.
During the mid-game, the enemy will be busy swapping lanes and setting vision. Therefore, as a Support, Gragas should work with the Jungler to invade the enemy Jungle and kill such targets.
Once Gragas gets to level 16, he will have his Ultimate R maxed out. It will give him a massive damage boost, and the low cooldown combined with the knockback will help him pick high-priority targets.
Gragas will either be highly tanky or a glass cannon dependent on landing his combo depending on the build. As a Jungler, Gragas will mostly be the latter, so his main goal will be to blow up the enemy backline when he gets the chance.
His dash and Ultimate R make up for a significant amount of peel for his carries. Should he go for the tank build route, he will successfully absorb a lot of damage for his team while his team follows up on Gragas' engagement.