.GM File Extension
File Type 1GameMonkey Script
What is a GM file?
Script written for GameMonkey Script, a scripting language used for embedding logic into games and other applications; uses syntax similar to the C language and uses concepts similar to the Lua language; can be used for custom software modifications.
GameMonkey Script files can be run with the GameMonkey Script engine and edited with any text editor.
File Type 2Transport Tycoon Music File
This file is saved in a binary format, which requires a specific program to read its contents.
.GM File Association 2
Music file used by Transport Tycoon (TT), a DOS-based railroad business simulation game; stores audio information using MIDI data and can be renamed with the .MID extension to play as MIDI audio; used for looping music while playing the game.
MIDI files are also used by OpenTTD, the open source follow-on version of Transport Tycoon, but OpenTTD uses the ".mid" extension for game music instead of the ".gm" extension.
NOTE: GM files are also used by Transport Tycoon Deluxe (TTD), the successor to Transport Tycoon.