Against gragas, Do not group closely together when sieging an objective or team fighting as it provides Gragas with an opportunity to knock your whole team into his with his Ultimate R.
 
When fighting, make sure you’re not too far forward or split from your team. If you’re too far forward, he could use his Ultimate R to push you into his team. Getting caught out like this could result in the enemy taking an objective shortly afterwards. 
 
Try to constantly poke and harass Gragas. Having him on low health will prevent him from playing or positioning aggressively. Against gragas, Don't regroup closely in an objective fight or team because it gives Gragas the opportunity to hit your entire team with his Ultimate R. In a fight, make sure you're not too forward or get away from your team. If you're too forward, he could use his Ultimate R to push you into his team. Getting caught like this could lead the enemy to take a goal soon after. Constantly try poke and harass Gragas. Having him in bad health will prevent him from playing or aggressively positioning himself.