Crytek's EULA surely contains some guidelines on this. Where did you look for "info on this" if not Crytek's EULA? Just because you say it's a gray area doesn't mean it's really all that gray.

Everything (every creative work) is covered by copyright by default. Unless you've been given permission to do this, then it's copyright infringement.

That's black and white. They only grey area is "will I get caught for this"