This page details answers to common questions you might have.
Q: What version of GameMaker is the engine made for?
A: Using GameMaker's "Import" function, it should work for any newer version of GameMaker: Studio above 1.4. (has not been tested for GMS 2.x)
Q: How proficient should I be in GML if I want to use it?
A: Intermediate knowledge is recommended, but most of the engine should be understandable if you have slightly-above-basic knowledge.
Q: Are there any guidelines I should respect?
A: Just the ones that ZUN has in place for derivative content.
Q: Can I use code from the engine in an independent project?
A: Sure, if you really want to. Credit would be cool though.
Q: What happened to Touhou MDP?
A: It's dead.
Q: Why should I use this engine instead of, say, Danmakufu?
A: If you just have one boss or stage in mind, then I'd actually recommend Danmakufu over GMTE, but for entire games, I think GMTE would be easier to use given that's the specific purpose it's been made for.
A: I refuse to answer this question.