Although if you look at FSFs major projects Emacs and gcc they do run on many OSs including non-free (e.g. Solaris, macOS, Windows, VMS and virtually anything that exists) and other free Unixes.
If you mean the Microsoft project called "GUIX": sure, it's rather restricted.
Guix, however, goes out of its way to support extensions at different levels of its architecture. Not only can you extend it with channels, local files, and with Guile expressions, you can also rewrite the dependency graph of your environment with package transformations, e.g. to swap out any instance of Tensorflow with a CUDA-tainted variant of Tensorflow from the Guix Science Nonfree channel, recursively.
I won't disagree that guix can be made to use nonfree packages. However, the claim:
>> Although if you look at FSFs major projects Emacs and gcc they do run on many OSs including non-free (e.g. Solaris, macOS, Windows, VMS and virtually anything that exists) and other free Unixes.
>> GUIX is much much more restricted.
appears to hold up; https://guix.gnu.org/en/download/ says "Alternately, GNU Guix can be installed as an additional package manager on top of an installed Linux-based system." (emphasis mine) and although there's no explicit statement, https://guix.gnu.org/manual/en/html_node/Installation.html implicitly reiterates that guix is only targeting Linux. Forget running on Solaris, Darwin, or NT; as far as I can tell guix doesn't even care about being usable on any open source unix other than Linux.
Edit: Although now that I think about it, they also target HURD... which is no less niche, but does at least imply that it could work on non-Linux unixen.
Unfortunately it’s also Stallman’s — and thus sadly by extension the FSF’s — entire shtick.