The greatest champs globally to counter Galio include Ziggs, Samira, and Lucian. These are all good at countering Galio with win rates over 59%. Even if these Galio counter picks are not used in the same position as you, it's not a good idea to take him into any of these matchups. They will most often win against Galio.
Yet, if the opposing team choses either Kalista or Heimerdinger, you should seriously think about selecting him, as he easily destroys these champs with win rates over 56% against each ofthem.