This is awesome.
Testing it now with an MP32L and so far (just 10 minutes into it) sounds quite good and balanced.

It also couldn't be a better timing to have this available now since I have to stow away all my MIDI stuff for a couple of years to come most likely so the small MP32L will suit my GM and MT-32 needs very well in the meantime.

v0.2 (2022/6/29)
Vol env decay/sustain/release of instruments setup complete.

v0.3 (2022/7/1)
It is now a multi-velocity layered sounfont.
Recommended sound engine: Fluidsynth or RgcAudio sf2/sfz player.
https://github.com/FluidSynth/fluidsynth/releases
VirtualMIDISynth and FSMP(bassmidi) don't seem to support multi-layered sf2.

v0.4 (2022/7/6)
Replaced some short samples and fixed some bad loops.
But the size of the soundfont increased to 302MB.

v0.5 (2022/7/9)
Replaced some short samples and fixed some bad loops. (284MB)
https://drive.google.com/file/d/1wdYpwoCka8r7 … iew?usp=sharing

There is a mp3 comparison between my soundfont v0.2 (single layer) / v0.3 (multi layer) / Real SC55.
v0.2:
https://drive.google.com/file/d/1qoxyiiTGg9cz … iew?usp=sharing
https://drive.google.com/file/d/13-w397LU9n8m … iew?usp=sharing
v0.3:
https://drive.google.com/file/d/1XxU16LKVk7xF … iew?usp=sharing
https://drive.google.com/file/d/1L15qr1vQUb_r … iew?usp=sharing
Real SC55 (RA90):
https://drive.google.com/file/d/1IKDFlQzUlDLq … iew?usp=sharing
https://drive.google.com/file/d/1Snzab7qQdVpt … iew?usp=sharing

I got some SC midi files from
http://shells.tw/~bill/midi/midi.htm
https://archive.org/details/sound_canvas_midi_collection
as test samples and packed into:
https://drive.google.com/file/d/1rJ8W7WYNQoPX … iew?usp=sharing
Real SC55 (RA90) version:
https://drive.google.com/drive/folders/1CIqN6 … er7DihVAuwInP3E

KORG i3 (X3R) GM Soundfont: (New)
v0.3 (2022/7/14)

v0.4(2022/7/16)
https://drive.google.com/file/d/1KHl3FQ7A5S9Y … iew?usp=sharing

"Doraemon.mid" played with v0.3:
https://drive.google.com/file/d/19yHOHX2qk--s … iew?usp=sharing
"Factory Demo - Korg AG-10.mid" played with v0.3:
https://drive.google.com/file/d/1YpRvflP-lF3j … iew?usp=sharing
https://drive.google.com/file/d/1Q9AZgTnarRts … iew?usp=sharing
https://drive.google.com/file/d/15tOTvBUidEzj … iew?usp=sharing

-----------------------------------------------------------------
My preferences for fluidsynth:

fluidsynth.exe -f fluidconf.txt -g 1 -a wasapi -m winmidi -o midi.winmidi.device=0 SC55_zzdenis_v0.5.sf2

Contents of fluidconf.txt file:

Heavy effects:
https://drive.google.com/file/d/1Fi3WnAxprymT … iew?usp=sharing
----------------
set synth.polyphony 160
set synth.reverb.active 1
set synth.reverb.level 0.6
set synth.reverb.room-size 0.7
set synth.chorus.active 1
set synth.chorus.level 0.2
set synth.dynamic-sample-loading 0

Slight effects:
----------------
set synth.polyphony 160
set synth.reverb.active 1
set synth.reverb.level 0.3
set synth.chorus.active 1
set synth.chorus.level 0.1
set synth.dynamic-sample-loading 0

I got it. Sounds almost identical to Sound Canvas VA or RLNDGM.sf2 in terms of balancing. However, it's so much quiet. Also you might want to rename the patches keeping step with GM1 patch list. Great work anyway!
By the way, here is a trick. especially Hi-hats and maybe even cymbals should be able to be choked. And if you are using polyphone soundfont editor, specify an exclusive class value to the 3 samples that stands for hi hats.
like shown here