GM is an industry-wide standard of voices and messages that are compatible among all synths and synth-related devices. GM2, from what I have read, will be the next industry-wide standard that will double the amount of voices and add more features to synths and synth-related devices. GM voices can be played on all synths. GM2 voices will be compatible with all future synths when the standard is implemented.

GS and XG are proprietary and not industry-wide standards.

Larry