Jim Williams is our Guru for gulbransen analogs here on the forum. I'm going to push this into the active list in hopes he will be online soon. I too have toyed with some gulbransens in the past with a B and a D, but I'm not familiar with the C that must have come in between. If you could add a description of the keyboards and pedal board (spinet or console/ 13, 25 or 32 pedals?)
If 32 pedals it is possibly a D and not a C? Where did you locate the identifying information on the organ? The old Gully's are very reliable and fixable. I gave away mine to make room for more organs, but I miss them very much. I'm sure you will enjoy yours after some repairs.
Thanks for your question. Some great photos of old gullys can be found on his website: http://www.jimmyandsharonwilliams.com/gulbransen/