It's pretty much a separate implementation. It reuses a version of the Nix daemon because Guix targets the same derivation format as Nix, but the whole machinery that produces derivation files is completely separate.
Guix compiles first class package definitions down to derivations, so while the idea of package management as a function is front and center in the Nix UI it is abstracted away in Guix, which instead builds up a tower of transformations from the Scheme DSL to Scheme builder scripts and derivations.
Guix compiles first class package definitions down to derivations, so while the idea of package management as a function is front and center in the Nix UI it is abstracted away in Guix, which instead builds up a tower of transformations from the Scheme DSL to Scheme builder scripts and derivations.