My understanding is that it's definitely not okay to use Greenboard for a tile backing. From what I've read it MIGHT be okay if everything is done just so and the grout sealed, but only a very small minority claim this, virtually everyone else says concrete type backerboard.

My understanding also is that USG says the following:

Panels are not intended for use in areas subject to constant moisture such as tub and shower enclosures
Does this mean it should not be used as a backer for a glue up tub/shower surround?

It's my understanding that greenboard was designed to be used in moisture areas like bathroom walls, but not specifically behind surrounds and/or tile.

If it makes a difference this is the greenboard from about fifteen years ago, not the newer type.