The best champions outside of lane to counter Gangplank include Kalista, Lucian, and Tristana. These are all good at countering Gangplank with decent win rates over 57%. Even if these Gangplank counter picks are not taken opposite your position, it's not smart to take him into one of these matchups. They will most often win against Gangplank.
Conversely, if the opposite team picks either Zyra or Neeko, you should strongly think about choosing him, as he easily dominates these champs with win rates over 52% against both ofthem.