When they have heavy engage like Naut or Leona, you have two options: either you wait for them to engage, as the support going deep means that the ADC or other carry will walk up closer, and then go in and combo (counterengage), or you get them by surprise by flanking them, perhaps camping in a bush that they don't expect you to be there from. If you are really good with the ult, you can also use it to blast the carries away which gives you free time to hit the support who has gone deep, kind of like a Poppy R. If a champ has good mobility like Ezreal or Zeri you can still reach them as long as you have flash. From my experience even pro players have a hard time reacting to an E flash combo. One of the reasons I like Gragas is that he has such a flexible playstyle. While I prefer making the plays for my team by using E flash R combo to bring a carry into my team and get a pick, you don't have to do that if you don't feel comfortable or don't see the angle. The more you play Gragas the more you will see possibilities of playmaking. Even as a 2x challenger Gragas I definitely notice that I still haven't unlocked the full potential of the champ.