It looks like something is wrong with first gear. It looks like the gears are contructed from two pieces that are supposed to be joined. Gear 2,3,4 are joined but gear 1 is not.
It's hard to comment without photos, but:
Two kinds of gears, rack gears which are linear and everybody else that are round. Never saw a transmission with a rack gear so all yours should be round. If one isn't, yes that's at least part of your problem. What about fifth gear? Also, what about bits of whatever is broken steaming around in the transmission and elsewhere?
I would venture to guess that it's tear down time, disassemble, clean out, inspect, bearing replacement and more. I don't know anything about glide shift transmissions but suspect they are more complex than a standard gearbox. At the very minimum, if you were going to do the work, a Workshop Manual would be necessary. Messicks Farm Equipment has parts diagrams online.