How to counter Galio
Let us take a look on how to beat Galio. We will look at champions who are strong against Galio and who are weak against Galio. Use win rate, gold difference and creep score difference at minute 10 to find the best 
Mid champion who counters Galio. You can already win in champion select with the right counters for Galio. If you want to know how to counter Galio then you need to take into account the best picks vs Galio and the worst picks vs Galio. This Galio counter data is based on matches played by Pros on patch 15.12 and previous ones. It is important to see whether Pros counter picked Galio or whether they played as Galio.
Champions used by Pros to counter pick Galio
Looking at the matches from Pros (see table Pros counter picked Galio) we can see that they picked 
Sylas most often to counter pick Galio. The matchup Sylas vs Galio occured 28 times. Galio has 8 wins and 20 losses in this matchup. Learn how to counter pick Sylas to get another perspective on the data.
Lane outcome for Galio
When looking at Galio counters you will notice that win rates alone are not conclusional. Therefore we provide an overview (see table Lane Outcome) where you can see the lane outcome of Pros who played Galio. The lane outcome of Galio consists of three outcomes: Win, Loss or Draw. Let's take a look of Galio lane and match outcome for specific matchups. Considering the lane outcome and the match outcome of a specific matchup, might enable you to find the right counter for Galio.
Conclusion
Analyze the available data and learn how to beat Galio. To master Galio take also a look on Galio builds and runes for Galio. Good luck, Summoner!