.gm6

The (.gm6) is a file extension used by the Game Maker software as the source file of a game. Game Maker 6.x is capable of reading .gm6 files, however, former versions of Game Maker (5.3A and before) use the .gmd file extension for storing the source of the games.

Compatibility
.gm6 files can be read by any version of Game Maker 6.x, for example, version 6.1 could read a 6.0 file, and 6.0 could read a 6.1 file (that is, if none of the new features of 6.1 are used). Game Maker 7 is also able to read gm6 files. However, .gm6 files are not compatible with older Game Maker versions, like 5.3A and before.