The difference between Guix and GuixSD isn't about alternative names for the same thing (like whether e.g. Ubuntu is "Linux" or "GNU/Linux"). Instead these names describe two distinct, but related, software projects.
Guix is a collection of programs for building, installing and distributing software (or other digital artefacts). It is specifically designed to work alongside alternatives, like apt, yum, etc. on many operating systems.
GuixSD is a particular OS distro, which uses Guix for all of its packaging, etc. If you use GuixSD, you're going all-in on Guix for everything.
The distinction between these projects is important, since Guix might be a good engineering choice for some project (say, a Web site backend, requiring particular versions of Apache, Python, various modules, etc.), whilst it may be a bad idea to switch the whole underlying OS to GuixSD.
As a concrete example, I've not used Guix or GuixSD myself, but exactly the same distinction exists between Nix and NixOS. I personally use NixOS, and I include Nix configurations in my own projects, so others can use Nix to install them with all of the right dependencies, etc. They don't need to switch distro to NixOS though; Nix will work on whatever system they already use (e.g. Ubuntu, OSX, etc. Though not yet Windows).
You call your OS Linux, RMS calls it GNU, I call it Bash, my wife calls it XFCE, her granma calls it "that one with the blue mouse" (and uses it without a problem).
But I really think that the name should be Bash, because that's what holds it all together.
Guix is a collection of programs for building, installing and distributing software (or other digital artefacts). It is specifically designed to work alongside alternatives, like apt, yum, etc. on many operating systems.
GuixSD is a particular OS distro, which uses Guix for all of its packaging, etc. If you use GuixSD, you're going all-in on Guix for everything.
The distinction between these projects is important, since Guix might be a good engineering choice for some project (say, a Web site backend, requiring particular versions of Apache, Python, various modules, etc.), whilst it may be a bad idea to switch the whole underlying OS to GuixSD.
As a concrete example, I've not used Guix or GuixSD myself, but exactly the same distinction exists between Nix and NixOS. I personally use NixOS, and I include Nix configurations in my own projects, so others can use Nix to install them with all of the right dependencies, etc. They don't need to switch distro to NixOS though; Nix will work on whatever system they already use (e.g. Ubuntu, OSX, etc. Though not yet Windows).