Gigapulse comes as part of GS3 and theoretically you can route audio back into it as well as using it for your Giga instruments, but I've never tried this.
It also comes as a VST, which is a separate product and there is no financial benefit (when you buy it) for owning GS3 :>(
I don't like it at all, but I think that's because I don't like any of the IRs. However, I do like Altiverb, and it would have been the only reason that I would consider getting a Mac. I say "would have been" because a Windows VST version is in the works, so I'll just wait for that.