A GMK file is a game development project created with GameMaker 7 or later. It contains several different types of data, including program source code, project settings, and references to resources used by the project. External resources may include sprites, sounds, backgrounds, paths, and other objects.

GMK files contain source code written in the GameMaker Language (GML). This code may be automatically generated using drag-and-drop actions or manually coded by the developer. Files that contain only GameMaker source code may have a .GML extension. NOTE: Both GameMaker 7 (GM7) and GameMaker 8 (GM8) save project files with a ".gmk" extension. However, the file formats generated by each version are slightly different. GM8 is backward compatible with GMK files saved by GM7, but GMK files saved by GM8 cannot be opened with GM7. GameMaker 6 saved project files with a .GM6 extension.