I just discovered GameMaker Community Forums moderator YellowAfterlife’s wonderful in-browser utility, GMLive.
I’d long been wanting YoYoGames to produce a GML console that I could type in a simple snippet of code that I could run to see what it does without the need of having to set up a project, build, and run it, similar to the REPL in Python, so I’m thrilled to learn that this exists. It’s very well done, and while not 100% compatible with GML, it’s very close, and very well documented.