i am not a lawyer but you mind find this wikipedia article a good read - https://en.wikipedia.org/wiki/Intellectual_property_protection_of_video_games - specifically this section-
"In the United States, the underlying source code, and the game's artistic elements, including art, music, and dialog, can be protected by copyright law.[10] However, gameplay elements of a video game are generally ineligible for copyright;[10] gameplay concepts fall into the idea–expression distinction that had been codified in the Copyright Act of 1976, in that copyright cannot be used to protect ideas, but only the expression of those ideas.[11] The United States Copyright Office specifically notes: "Copyright does not protect the idea for a game, its name or title, or the method or methods for playing it. Nor does copyright protect any idea, system, method, device, or trademark material involved in developing, merchandising, or playing a game. Once a game has been made public, nothing in the copyright law prevents others from developing another game based on similar principles.""