Here are a few things you can reference:
1) The PDF for RedGard.
Reference 3), Suitable Substrates. There is no mention of any type of gypsum-based wall board being suitable. For clarification, it mentions "gypsum based cement topping" but that is gypcrete, a lightweight concrete-type mix. It is in no way related to gypsum wallboard.
2) I'm no expert on USG gypsum panels. This will probably come across as being a bit negative, but the reason is that they continually change the makeup and name of the different boards, pass them off as being suitable for certain applications, then when they fail 10 years, 25 years, or 20 years down the road, they reformulate the product, rename the panel, and start the game all over again.
I recommend you call USG and ask. I'm fairly certain that the MOLD TOUGH series is considered by the manufacturer as NOT SUITABLE for tub and shower surrounds.
If USG doesn't say that they are not suitable, but defers to TCNA standards, then they are not suitable.
In my world? It's not suitable. I'm a basics kind of guy. Cement board. Wonderboard or Durock in wet areas.
Good luck with your project.
USG contact info: for technical assistance please contact 1-800-USG4YOU (874-4968)
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Shower redo
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