Don't know exactly what you mean. There are no such things as GM or GM2 "files". Are you talking about Standard MIDI Files type 0 or 1?
GM and GM2 are standard sets of sounds and controls to be used on sound sources. GM2 e retro compatible with GM . It just adds some extra control functionality and banks of sounds. So if you use a song file writen for a GM sound source it will play in any GM2 sound source. If you want to use the before mentioned added functionallity and banks of sounds you just change/add this information in the song file.