The greatest champs globally to counter Galio include Ziggs, Samira, and Lucian. These champions are all great at countering Galio with win rates over 59%. Even if these Galio counter picks are not used in your lane, it's not a good idea to play him into any of these matchups. They will very likely crush Galio.
Yet, if the opposing team choses either Kalista or Heimerdinger, you should strongly consider choosing him, as he easily destroys these champs with win rates over 56% against each ofthem.