The driver software didn't really get involved in MIDI playback for the first few cards. Remember that this was mostly DOS and every program was responsible for its own drivers and instrument definitions. Only when Windows 3 became the de facto standard did the manufacturers of the cards write drivers that mapped MIDI to their chips. Windows 3.x had no concept of GM, just of MIDI itself. All it was concerned with was mapping MIDI channels onto drivers. That is what the MIDI mapper did. So, for DOS there was no MIDI driver provided by the card manufacturers. Sequencer software publishers made their own, if they supported FM sound at all.
Windows 3.0 is older than GM and Windows 3.1 was later but, as mentioned above, didn't actually need to know GM. The drivers did. I'm not sure whether FM drivers for Windows, pre-dating GM actually provided their own instrument lists, but I somehow doubt it. Windows 3.0 was the first somewhat successful version of Windows and 3.1 was a megahit. Only at that time, when Windows became a serious contender, was it really interesting for manufacturers to start building drivers for Windows -- that happened after 3.0 became successful, and GM existed by then. Before GM and .MID files became commonplace, .CMF and .ROL file were the most common for FM music, which included their own instrument definitions.
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