This old unit does not have a factory reset procedure because it did not have the patches stored in a ROM where they can be recalled from.
Therefore the only way this can be achieved is by bulk uploading a SYX file with the presets through MIDI.
People keep asking me to mail them those factory presets, so I decided to share them here.
The procedure is:
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1) Make certain that the toggle switch on the back of the unit is set to MIDI.
2) Press the MIDI button so that the display reads)[MIDI CHANNEL /
CHANNEL = ?].Use the dial to set the MIDI channel to say [1].
3) Play the SYX file in from the MicroComposer, and the LED display will
change to [Ld], indicating that the unit is receiving sys ex data.
Instead of MicroComposer you can use almost any other MIDI player. I have personally never seen this software. I used MIDI-OX myself.
The important thing is to set the MIDI channels right.
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O

K this is an update:
Cathbard was kind enough to host the factory presets file on his site

. Here is the link where you can download it from:
http://cathbard.com/files/gp8_orig.mid
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Here is another update:
I finally set up my website. There is a page on GP-8 there with the factory presets, the manual and some comments.
Check it out:
http://www.andreinski.com/