25. What is the best way to thank your GM?
Say "thank you".
OK, slightly less sarcastic answer incoming.
There are a lot of great ways to show your appreciation to your GM. The best is probably by being an enthusiastic participant in their games, putting as much of yourself in them as the GM does. That shows you care in a way that bringing pizza doesn't, although pizza and beer never hurt.
Being a GM can be a lonely job, so seeing commitment and energy from your players is very energizing. It shows that they're on board, they're digging it, they're bringing stuff to the table that makes the game better.
It's always helpful to hear people talk about the game afterwards, or at other social gatherings, because it's a nice ego boost when people say nice things, and constructive criticism or ideas about where the game could go next are always helpful. Honestly, sometimes it's like pulling teeth to get any feedback at all, and specific feedback especially. Communication helps everybody.
The basic responsibilities of a player -- showing up, knowing the basics of the rules, paying attention to the game, the GM, and the other players -- are essential to the health of a good game, and doing at least those things is something every GM would be grateful for. Going above and beyond to make special contributions to the game -- now we're talking about stuff that really makes a game memorable. That could be doing a piece of art, or making a playlist, or writing detailed recaps, or volunteering to make a meal or dessert or something extra special.
And it's always lovely when people bring a bottle of wine or snacks.