Thanks for all the feedback, guys. I'd like to try mapping the controls to an Xbox controller to see how it feels, but I don't know if I can do that in GM8.
You can, but I remember needing an XInput DLL or plugin/add-on/whatever in order to use the triggers independently.
Is there a reason you're not using GM:Studio? Games seem to run much, much faster with it and there are some useful new features (including better support for gamepads). The Standard version is free but has an annoying "Made with Game Maker" splash screen (probably fine for prototyping, but maybe not for a full release). Luckily, the Pro version goes on sale pretty often. I think I got it for $25 or something.