According to the law you actually cannot do that.
You want to protect the game rules, rules are inherently ideas, and ideas cannot be protected, only your text can be copyrighted (code and GDD), the name trademarked, and that is it.
The patent law says it is illegal to patent ideas, including software and game rules (unfortunately patent offices ignore this, so yes, you CAN try to patent your game, and try to enforce the patent, but it may, or may not work).
But that is for obvious reasons, if your concept is THAT great, then it is good for humanity to it be copied, it is up to you to get money and credit with it (you can for example create a exceptional game, and everyone will call the genre as "yourgame-clone" or you can name the thing after yourself, like Perlin Noise...).
Ever tought of what would happen if id Software could prevent anyone else from making FPS, and EA from anyone making RTS? Or the MUD people preventing anyone from making MMO?