I think the same. GM started approximately 1993. Before, there was MT-32 (and compatibles). And generic MIDI, as well.
Some Sierra games also supported popular Casio keyboards of their time (MT-540/CT-460/CSM-1/CT-660/..),
as well as the IBM Music Feature Card / Yamaha FB-01.
Anyway, there was no definite break, I think. It rather faded from MT-32 to GM.
Some games released 1993 onwards also still contained MT-32 support:
Princess Maker 2, Sam&Max, Lollypop, StarTrek 25th anniversary/Judgment Rites, MegaMan X, etc.
Edit: That beeing said, there's nothig wrong in adding a MIDI board to your PC.
If you're playing some games like Descent or like listening to MIDI music, a simple/cheap GM daughterboard is a nice addition.
Also, a few pre-1993 games had generic support for MIDI synths (Piano on channel 1 was common even on pre-GM).
Long story short: It's up to you. A DreamBlaster synth or one of the Terratec daughterboards can be found for little money.
Adding something valuable like a DB50XG might be a bit overkill for a slow machine, though.
On the other hand, that one is said to have some sort of an MT-32 emulation "mode" (MT-32 instrument mapping). Hmm. Tricky. 😐
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