I imagine you'll shortly get bombarded with replies from ex-V crews and nav inst techs. I don't know too much about the workings of the kit as I was primarily a radar/NBS tech (on Vulcans), but what I'm pretty sure about is that the Vulcan had the GPI mark 6. I would therefore imagine the rest of the V-fleet had the same GPI. If I'm wrong, I expect Pontius Navigator will correct me.
Try this for a little bit of info on the GPI6
http://john-dillon.co.uk/V-Force/nav_kit.html
MSJ