Hello, original author of the PR you linked here. You can, in fact, install GCM through my NUR (where I’ve already added the package as the PR got lost in the thousands of open PRs on nixpkgs). NURs are basically alternative repositories by individual users, and often have more obscure or customized packages. It’s listed here, and you can see instructions for using NURs here.
The package would then be nur.repos.utybo.git-credential-manager. You can then follow the instructions in the PR for using it with Home Manager, but with the correct package, i.e.:
programs.git = {
enable = true;
# ...
extraConfig = {
credential = {
credentialStore = "secretservice";
helper = "${nur.repos.utybo.git-credential-manager}/bin/git-credential-manager-core";
};
};
};