I just noticed while I was watching the Jeremy Clarkson "Good Bad & Ugly" DVD that the prototype GTX1 (painted in orangy-yellow), has the "prototype gas cap". Must of been one of the first cars built, as no other GTX1 appears to have the "prototype gas cap" in all the photos I could find.
I was going to call Genaddi and ask them if they were producing the cap, but then I took a close look at some blown up photos of the original (red, no roof stripe) donor car, and it had the prototype gas cap on it when it was delivered to Genaddi.