.gmd
From GMpedia.org Wiki
.gmd The (.gmd) file extension is the extension that is used for source files for previous versions of Game Maker, starting with Game Maker 4.0 until Game Maker 5.3a. Newer versions of Game Maker (6.x) use the (.gm6) extension instead.
[edit] Compatibility
Game Maker 6.x is capable of reading both .gm6 and .gmd files, but Game Maker 5.x and before are only capable of reading .gmd files.
