Previously I was talking about a Casio LD-80 while playing through MIDI input reveals that it responds to GM as of DREAM SAM9733 tone generator.
The DD-50 manual doesn't list all the GM compatible voices (except the Drum Map). Now I searched for "DD-50 MIDI voices" but came up empty thus nobody shared the list yet and somehow I kept looking using terms "DD-50" "GM" then found this in French back In 2009 in that French Audiofanzine forum post someone tried to list the DD-50 voices, but the list may be wrong (e.g. mapped instruments in wrong instrument numbers are blank but some are alias).
I am trying to get a DD-50 from an online store and when it comes I'm going to try it out and do my best to list them. Or if anyone has a DD-50 and has navigated through 100 (or less) Voices via MIDI In then documented them that would be great.
UPDATE: My DD-50 I ordered from Mercari has arrived, so I decided to try experimenting from my computer the MIDI input features while scrolling though most of the program changes and found that interesting to do.
And here it is, a list of 33 of the 100 GM-mapped backing voices for the DD-50 (Bank MSB 0) all done by me, all of those names may be chosen by me. For those who want to use this as a reference while using the DD-50 as a MIDI sound module or know what voices on the DD-50 would change like before or when they have one.

1PC#		Voice Name
2[4]		Electric Piano 1
3[5]-6,0-3	Electric Piano 2
4[7]		Clavi
58-[10]		Music Box
6[11]-14		Vibraphone
7[16]		Electric Organ 1
8[17]		Electric Organ 2
9[18]-20		Rock Organ
10[21]-23		Accordion
11[24]		Acoustic Guitar 1
12[25],15		Acoustic Guitar 2
13[26]		Electric Guitar 1
14[27]		Electric Guitar 2
15[28],31		Mute Guitar
16[29]-30		Overdriven Guitar
17[32]		Acoustic Bass
18[33]		Finger Bass
19[34],38-39	Pick Bass
20[35]		Fretless Bass
21[36]-37		Slap Bass
22[45]-47		Pizzicato Strings
23[48]-51,40-44	String Ensemble
2452-[54]		Synth Voice
25[55]		Orchestra Hit
26[56]-60		Trumpet
27[61]-63		Brass Section
28[64]		Soprano Sax
2965-[66]-67	Tenor Sax
30[68]-71		Oboe
3172-[73]-79	Flute
3280-[81]-96	Saw Lead
33[96]-98		FX
34[99]		Sine Lead

The numbers in a bracket indicate the proper voice # (which are used as transmit in one of the songs in the DD-50), numbers outside the bracket are alias of that voice.
Program numbers in the range 100-127 are ignored.
Selecting Bank MSB 1-126 (LSB is ignored) will be a silence (turns voice OFF).
Some of the Voices (such as the ones with stereo chorus like Electric Piano 2, Electric Guitar 2, etc.) use up 2 notes (making it 14 note polyphony rather than 28) rather than 1 note usage on other Voices (while the MIDI Input key range is 12-96 also some voices respond to MIDI notes 12-23, others that don't will not sound in that key range).
The note map of the 4 Drum Kits (0, 16, 25, 32) are the only ones listed in the DD-50 manual, not the backing voices.
The GM Reset SysEx is not supported, so you have to send each CC (Bank Select MSB, Volume, Pan, Expression, Reverb Level, Pitch Bend Range, Channel Fine Tune, Channel Coarse Tune, and Reset All Controllers) and PC(#0) on every MIDI channel to its initial values.