Game development project created with Game Maker 7 or later; contains source code, project settings, and references to resources used by the project; resources may include sprites, sounds, backgrounds, paths, and other objects.
GMK files contain source code written in the Game Maker Language (GML). This code may be automatically generated using drag-and-drop actions or can be manually coded by the developer.
NOTE: Both Game Maker 7 (GM7) and Game Maker 8 (GM8) save project files with a ".gmk" extension. However, the file formats generated by each version are slightly different. GM8 is backwards compatible with GMK files saved by GM7, but GMK files saved by GM8 cannot be opened with GM7. Game Maker 6 saves project files with a .GM6 extension.