I was wondering this myself. Bought mine used and thought I'd swap the worn belt before it breaks so I can compare the size to the Gates 120''. I've read a thread here about troubles with correct size belts. Did anyone reply with the answer to the gearbox question?
Mine is a Ford 930B finish mower, and I don't remove the gear box to replace the belt. It has been a while since I replaced the belt and don't recall exactly how. Mower and manual are in Michigan. I am where it is warm. If you do not receive any other replies, PM me in May and I will send you instructions from the manual.
Typically, there is space between the mower deck and the pulley beneath the gearbox, which would allow you to slide the belt under the pulley to install. But, if removing the gearbox is handy, it is always a good time to drain and replace the gear oil.