I just published my first asset, GMSSimpleMIDI. It lets you send and receive MIDI messages in Gamemaker Studio, letting you make musical sequencers or arpeggiators with Gamemaker, or control your games with MIDI hardware controllers. It's on the marketplace and itch.io, and the source for the DLL itself is freely available on Github if anyone wants to recompile it for use with OSX or Ubuntu - it should work right out the box with either. I also made a quick demo with the extension, of a simple arpeggiator that receives MIDI notes and clock (or uses an internal clock) and transmits notes. It's a weird overlap of electronic music and game creation that I'm not sure too many people would be interested in, but I'm happy to answer any questions about it!