The problem (especially pre-flakes) with nix (and Docker too; haven't used Bazel) is that if you GC, you can't recreate your previous environment (which version of nixpkgs were you using when you built it/which day did you run apt-get update in your Dockerfile).

It appears that Flox uses flakes and versions the flake.lock file, so it should be possible to readily reproduce anything that hasn't completely disappeared from the internet.