I love the Honda GB at the moment I've got 2 of them, great bikes but quite hard to source any parts for them here in Thailand, but there are heaps of spares that can be ordered from Japan. The GB 500 is harder to find in Thailand as it was not meant for the Asia market.
The engine on the Honda GB clubman is indeed like an Xr but other than the engine the rest of the parts are all unique to the GB. In Australia and the UK the GB engine is used in a different frame and is called XBR500 and so the XBR workshop manual( part # C2008812F) Published in 1985 can be used for the GB because according to my research Honda did not produce a workshop manual for the GB in English. Also the GB comes in two versions one been a single seater cafe racer version complete with front fairing and no passenger footpegs and the other having no front fairing, but a double seat and passanger footpegs.
Dave, there are quite a few that appear on there in and around the area of CM if you want to know anything more just give me a call ... Rich I will tell you my experiences of my time as a fan and owner of the GB 4OO .. and I myself are at the moment trying to find a a spare tank, supertrapp exhaust, a 250cc engine, and a set up of passanger pegs.