So it is Les Paul with a Roland GK pickup built-in? Is it factory fitted or aftermarket?
BTW, the Roland GR-1 is an older floor synth unit, not the guitar itself. It is a good unit, but the sounds are somewhat dated compared to more recent models. You can buy a GR-1 and a pickup for around $300.
As far as the guitar, if the GK pickup is mounted with screws, that would generally devalue it compared to a non-GK Les Paul. If it is mounted with double stick tape (and if you can remove the unit without any finish problems) then the guitar would be valued like a regular Les Paul, and you'd add the value of the GR-1 and GK pickup.