Are the walls and ceiling going to receive tile or just paint.
Green drywall is junk in my opinion and no worth any premium over regular drywall. Invest in a good primer (Zissner Cover Stain) and excellent bathroom paint if you are going that route. Drywalling ceilings you should work with a ceiling drywall or 12" center joists.
If your setting tile on those walls 1/2" cement board or 1/2" hardie work great - so does Green EBoard. Green EBoard is not green drywall.