What is the best way to thank your GM?
Be present. Contribute. Communicate.
Before the session, commit to the game. If you can’t, then pass. Early.
During the session, don’t just turn up on time, but be attentive and active when it’s game time.
Be proactive to share in the storytelling. Be responsive to build on the group fiction. Not just what the GM feeds in, but the other players as well.
After the session, tell your GM what you enjoyed. If something happened that broke the game for you and was within their gift to change, offer feedback.