GMK is an extension specific to files related to a cross-platform game development environment called GameMaker. GMK files are used to store game designs made in the program. Such a project includes the source code, complete information about the game being developed and the location of all external resources used by the game.

Although GMK files are considered programming files, GameMaker also allows users to create projects who do not code in a traditional way, but use other mechanisms available in the program (e.g. drag-and-drop method).