.gmk

The (.gmk) is a file extension used by the Game Maker software as the source file of a game since Game Maker 7. Unlike .gm6 files, .gmk files are encrypted.

Compatibility
.gmk files can be read by Game Maker 7.0 and Game Maker 8.x. Game Maker 8.x can read .gmk files made with Game Maker 7, but Game Maker 7 cannot read files made with Game Maker 8.