Now, this is interesting. I have the makings of three GB250s that I bought cheap a few years ago. As usual, I still have done nothing with them. They appear to have been imported direct from Japan into southern Ireland. My understanding is that the GB250 was a Japanese only market bike, so is a Grey Import anywhere else. There was a mid 80s version, and an early 90s version. Both differ a fair bit. Also, one model is single carb, and the other twin carb. The engine is the same as the Honda CBX250, overhead cam, 31 bhp, which was a fairly sporty 250 in the 80s, designed to replace the fantastic CB250RS. But, they did not sell well at all, and I'm guessing that that is due to the change in learner laws down to 125cc.
Cant remember much more. Tim