GS in MIDI consists of all the General MIDI patches (sounds such as
flute or trombone.) plus others accessed by extra banks. I've used
these extra banks in my files mainly to avoid "flanging" which occurs
when the same note in the same patch is used in different channels at
the same time.
This is especially useful in the music of Elgar where the
composer uses many doublings and where reducing these doesn't give the
desired effect. When played on a GM sound source. the flanging will
occur but the sounds will default to a single bank and will usually
give a not-too-bad reproduction of the file.
32 channels (erroneously called "tracks") again are useful
in Elgar and other late romantics who use really large orchestras, but
these can be reduced to 16 channels in most cases. I haven't done this
in the Elgar 2nd Symphony but the others are available in 16 channel
versions on this site (see Music) and in
Classical Archives. For other sites, see Web Links.
The last Elgar work that I sequenced was the Violin
Concerto. This was more lightly scored so as to not drown out the solo
violin and I was able to do it in 16 channels from the outset.