Do you have to go through Humble to activate your copy or YoYo Games?
I can’t say for certain, but I believe that you can always download it from YoYo Games’ site and just use your activation key when you install it. If you have to go through Humble you should probably go ahead and redeem it, just in case.
GameMaker has some tutorials included in the download when you start a project (you will see them in the tabs at the top). There are also a few tutorials on YouTube of course, but admittedly I haven’t spent enough time with either since Unity better suited my needs at the time.