How to counter Gnar
Let us take a look on how to beat Gnar. We will look at champions who are strong against Gnar and who are weak against Gnar. Use win rate, gold difference and creep score difference at minute 10 to find the best 
Top champion who counters Gnar. You can already win in champion select with the right counters for Gnar. If you want to know how to counter Gnar then you need to take into account the best picks vs Gnar and the worst picks vs Gnar. This Gnar counter data is based on matches played by Pros on patch 14.9 and previous ones. It is important to see whether Pros counter picked Gnar or whether they played as Gnar.
Champions used by Pros to counter pick Gnar
Looking at the matches from Pros (see table Pros counter picked Gnar) we can see that they picked 
Renekton most often to counter pick Gnar. The matchup Renekton vs Gnar occured 28 times. Gnar has 16 wins and 12 losses in this matchup. Learn how to counter pick Renekton to get another perspective on the data.
Lane outcome for Gnar
When looking at Gnar 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 Gnar. The lane outcome of Gnar consists of three outcomes: Win, Loss or Draw. Let's take a look of Gnar 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 Gnar.
Conclusion
Analyze the available data and learn how to beat Gnar. To master Gnar take also a look on Gnar builds and runes for Gnar. Good luck, Summoner!