How to counter Gragas
Let us take a look on how to beat Gragas. We will look at champions who are strong against Gragas and who are weak against Gragas. Use win rate, gold difference and creep score difference at minute 10 to find the best 
Top champion who counters Gragas. You can already win in champion select with the right counters for Gragas. If you want to know how to counter Gragas then you need to take into account the best picks vs Gragas and the worst picks vs Gragas. This Gragas counter data is based on matches played by Pros on patch 15.13 and previous ones. It is important to see whether Pros counter picked Gragas or whether they played as Gragas.
Champions used by Pros to counter pick Gragas
Looking at the matches from Pros (see table Pros counter picked Gragas) we can see that they picked 
Aatrox most often to counter pick Gragas. The matchup Aatrox vs Gragas occured 21 times. Gragas has 11 wins and 10 losses in this matchup. Learn how to counter pick Aatrox to get another perspective on the data.
Lane outcome for Gragas
When looking at Gragas 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 Gragas. The lane outcome of Gragas consists of three outcomes: Win, Loss or Draw. Let's take a look of Gragas 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 Gragas.
Conclusion
Analyze the available data and learn how to beat Gragas. To master Gragas take also a look on Gragas builds and runes for Gragas. Good luck, Summoner!