There is a little bit of info easily found on them with a quick Google search. I found this posted in Yesterdays Tractors in response to a similar question to yours ""I have a fair amount of information regarding GBT tractors. I don't have e-mail at this time. My phone no. is 419 985 3891. My adress is 1375 ST RT 294 Marion Oh 43302.I have a GBT 3000 that I am restoring. I also have sales bochures and some prices from 1977.I also have a letter from the designer of these tractors.I also had an article in the Feb or March 1999 Gas Engine magazine. Call or write and I'll share information.Lowell Cooper""
Possibly this individual could point you in the right direction?