packageOverrides = pkgs_: with pkgs_; {
    testgcc2 = pkgs_.gcc;
  };

% nix-instantiate '<nixpkgs>' -A gcc
warning: you did not specify `--add-root'; the result might be removed by the garbage collector
/nix/store/993glmz6hxbld0ry81jwv7c3z7cgaz68-gcc-wrapper-4.8.2.drv

% nix-instantiate '<nixpkgs>' -A testgcc2
warning: you did not specify `--add-root'; the result might be removed by the garbage collector
/nix/store/41nacbfpr8jzsk3k03jjlqb2r2xis5gf-gcc-wrapper-4.8.2.drv