Well, if it appeared alongside the GS Wavetable Synth, it would be a MIDI device. Then any Windows MIDI-based application would support it - or be able to make some sound through it at least
If the OP just want to be able to play MIDIs using FM sounds, I suggest this: http://www.geocities.co.jp/SiliconValley-SanJose/8700/P/GsorigE.htm
The program starts up in SCC mode (based on the sound of the MSX home computer), but clicking the GXSCC logo switches to FM mode. It can be great fun playing MIDIs with this player.