Waterproof front lip of shower pan that is beveled for entry?
If you caulk the hell out of the bottom edge, and construct the wall so it doesn't move relative to the shower base, then you should be fine. A rubber gasket will probably be less effective than caulk, since caulk can be made to fit any shape of crevice.

Do not put grout in the line between tile and base, use caulk instead. Same goes for the corner seams between the walls. Many grout manufacturers also make caulk that matches their grout colors, so the transition will be invisible. Caulk is flexible, so it can handle slight movement without cracking.