Hello, sully!
I'm happy to help with any questions through my Discord server, which is on Itch.io. There's also a Getting Started guide that will help you add Crystal to your game.
To make you comfortable: I'm one of the developers who best understands how graphics work in GameMaker, so you can rest assured.
Crystal was designed with pixel art in mind as well, so depending on what you've done, it should be compatible. But if you're interested, I have another library called Rezol that handles pixel-perfect from the box and works well with Crystal as well.