create pkgs folder in /etc/nixos/
inside /etc/nixos/pkgs/ create a gamingtool folder.
create default.nix in /etc/nixos/pkgs/gamingtool
add (pkgs.callPackage ./pkgs/gamingtool/default.nix {}) in your configuration.nix
sudo nixos-rebuild switch
Gamingtool: https://github.com/FinGu/gamingtool