It was a bit more than just a guitar synth. It was the world's first (only?) Digital sampling guitar. Steve used it on one track on his "Momentum" tour in 1988, which I was lucky enough to see at Manchester Opera House. He set up a tape loop with an electric guitar (years before KT Tunstall was doing it, and decades before Ed Sheeran got himself in unjustified lumber with it! ) then picked up the Stepp DG1, which looked a bit like a transistor radio (a high end one like a Sony ICF-2001D or a Grundig Satellit 700) with a fretboard, and proceeded to create what can almost be described as a religious experience! You could assign different samples to each string, so one had female choir, another male, another organ, and so on.
The opening of "Concert for Munich" was it's first appearance on a Steve Hackett record.
Stepp marketing video: