There is no way to disable the orange key in the firmware, so if it is not working it is probably broken.
A few other people have had the same problem. I suspect the switch is not very robust.
Colin G4EML.
GD-77 Orange button (SK3) not working
Re: GD-77 Orange button (SK3) not working
I think Jason VK7ZJA may have a modification for this fault on his website
Try searching for something like "GD77 mods" and VK7ZJA
Try searching for something like "GD77 mods" and VK7ZJA