I came across this document on the Roland website yesterday while looking for GS specs.
It's Roland's Introduction To MIDI. I found this part particularly interesting, as I did not know this until now.
General MIDI solves this since all General MIDI devices contain the same set
of 128 standard sounds and drum kit sounds, stored in a specified order.
Each product creates these sounds using its own unique capabilities, but the
goal is to make them all sound like the sounds in a Roland SoundCanvas.
The General MIDI sounds originally came from Roland’s SoundCanvas
sound module. These sounds did such a good job of meeting musicians’
needs they became the foundation of the General MIDI standard.