Guix System is the NixOS equivalent. It is a GNU FSDG compliant, rolling release, non-systemd GNU/Linux operating system.
Guix System also has a much smaller community compared to NixOS (also more than a decade younger), but it is a community of passionate hackers who work to improve the distribution one way or the other. The big difference of course being that it is all written in Guile Scheme rather than a Domain Specific Language. So you’ll learn about things like g-expressions (Guix’s take on LISP’s s-expressions), Guix services (system and home), and Guile Scheme itself.
If you want to work with Guix, you basically have to become a contributor to the project, but it’s really a rewarding experience if you choose to do so.