I think the GS synth really relies on the .idf file. Anyone know exactly what it does?
I have somewhat of an idea of its function. From what I know, Windows refers to that file when sending information to a MIDI device. Appearantly, back in the days when GM was totally new, and having 16 channels was hi-tech, there were other modes, called "Extended MIDI" and "Basic MIDI" (I'm foggy on whether or not "Basic MIDI" is the correct term), that determined what channels the synth had available. "Extended MIDI" used channels 1-10 only, and "Basic MIDI" used even fewer channels, like only 4 or 5.
Anyway, it is also used when clicking the "Add New Instrument..." button in the MIDI tab of Multimedia properties. After you tell windows what port the MIDI instrument (its really supposed to be for when you need to manually tell windows that you've got an electronic keyboard hooked up) is connected to, and then it asks you for the .idf file, and in that case, the General.idf should work fine unless the keyboard came with its own custom ".idf" file that windows should use.