let
  unstable = import <nixpkgs> {
    overlays = [
          (import (builtins.fetchTarball {
            url = https://github.com/nix-community/emacs-overlay/archive/master.tar.gz;
          }))
        ];
};
in
{
  environment.systemPackages = with pkgs; [
    unstable.emacsPgtkGcc
  ]
}

---


error: anonymous function at /nix/store/kxj21dpw1az5pl0y2jlaxgjj4b1ynhwj-nixos-20.03.3316.0f8a31b992b/nixos/pkgs/applications/editors/emacs/default.nix:1:1 called with unexpected argument 'nativeComp', at /nix/store/kxj21dpw1az5pl0y2jlaxgjj4b1ynhwj-nixos-20.03.3316.0f8a31b992b/nixos/lib/customisation.nix:79:63