The greatest champions globally to counter Gragas include Caitlyn, Zeri, and Jinx. These picks are all good at countering Gragas with high win rates over 67%. Even if these Gragas counter picks are not picked in the same position as you, it's not a good idea to choose him into these matchups. They will likely destroy Gragas.
Conversely, if the opposing team choses either Swain or Ryze, Gragas mains should strongly think about picking him, as he easily wins against these champs with win rates over 52% against both ofthem.