Yes, Gore-Tex is waterproof yet it breathes. I believe it's also lighter than a plastic tarp. These are some of the reasons we're interested in using it. The cost is much greater than a tarp, but we'd like the inside of the dome to be tolerable even during a daytime rainstorm.

A rainfly is exactly what we're planning to use it for.

What I've heard about plastic tarps is that they're certainly wind and waterproof, but they don't breath and so under the right weather conditions it can get hot and muggy in the dome.

I have a lead on some reasonably priced Gore-Tex fabric that we're thinking of using to cover our dome. Anyone have any good reasons why it might be a bad idea to use Gore-Tex? Any feedback is welcome.