Designing Kitchens and Bathrooms - Tub surround, Tile on Greenboard

Before I start, yes I know I screwed up to begin with, should have used cement board over poly vapor barrier, but I did not know that at the time. Question is can I make a fix that will not cause major problems.

I am putting tile in my tub surround over greenboard I just but up. The area has been taped and floated and is all nice and even. Can I use Redguard or something like it to waterproof the area rather than having to tear out everything I have done so far and start over?

Please help this project has been frustrating enough and the prospect of start the tub part over is NOT exciting.

Is this a shower as well, or just a tub?

If it's just a tub, then you should worry to much about it. If it's a shower, I'de say adding Redgard will definetly help. Redgard does say you can use it over gypsum board, which greenboard is, but not sure if it's ever been specifcally tested for greenboard.

I would apply at least 2 coats of redgard, following their instructions to get the required thickness.

It is a shower as well. When I found Redgard on the internet the specs talked about Gypsum concrete substrates, I could not get hold of the company... any experience with green gypsumboard (or is that the same thing?)