It is unfortunately not possible to make a simple adaptor cable to interface the Soldano Decatone and the G-Major. The Decatone footswitch input is operated by three momentary switches and the G-Major has "only" two alternating relays.
The possible solutions would include either a box with logic circuitry + power supply or a modification of the amp.
The solution below was suggested by Bill Sundt from Soldano:
"..unfortunately the G-Major only has two relays and the Decatone would require three. In theory it could be modified to work with only two, but we have not actually tried this, and it would be fairly complicated, especially after the amp is already built. Also, the decatrol would then no longer work, unless the mod were further complicated by the addition of a switch to select for the switching device.
As an alternative, there was a company named Axess Electronics, which made a small midi controlled switcher with four relays. It could be made so that it would plug right into the switching socket on the decatone, using the MIDI cable. (Note that this socket is not actually a MIDI jack, it only uses the same socket, so that a 5 conductor MIDI cable may be used. Do not plug an actual MIDI device into this socket, it will likely damage the MIDI device).
We have not actually seen or used one of these devices, and were informed about them by another Decatone owner, who was inquiring if we were aware of the company.
Axess Electronics appears to have gone out of business in 2009, so the 2nd hand market would be the only way to purchase one of their products.