Does RedGard count as a rubber sealer in a shower?
Yes that is what RedGard is for (I assume you are planning to tile over the top of it).  It is a waterproof membrane, it stops any water that might make it through the cracks in the tile (or grout).  It is not intended to stand up to the abuse of a normal shower wall or floor so it must be covered with tile.
From the company's website:

Rated for extra heavy duty service
Meets Uniform Plumbing Code specifications for use as a shower pan liner

I used RedGard to waterpoof behind my tiles in my tub surround.  It was pretty easy to work with.