I'm not sure if this has been asked before but, i'm trying to use gamemaker and GML to make a game, however for scope and modding reasons I'm trying to make all stuff outside of gamemaker. My issue now has to do with programming items, enemies, and bosses outside of gamemaker. Is there a way to use GMEdit to create gamemaker objects and write GML outside of gamemaker?

GMEdit is a code editor that you can set up to display auto-completion for your game-specific API.

For implementing a custom scripting language, see Tiny Expression Runtime.