If the JTD can take a GL5, then you ought to stick a GL5 in there.
I've used Motul Motylgear 75W80 in mine in the past, which Motul refers to as a "GL4/5" ... which is unhelpful given the info' in Jim's post.
It's not as smooth as whatever was in there before but then I think my clutch or flywheel is probably on the slippery road to Boot Hill, so it might not be the oil's fault.
When I swapped a driveshaft I topped it up with some Comma SX75W90 which is GL5. Comma doesn't do a 75W80 in GL5 flavour... and it feels/acts the same as before. It was a quite substantial top-up of the Motul, but not a complete replacement.
If it's winter where you are, and it's -0C or colder, use a 75W80. If you can only find GL4, then GL4 is better than no oil at all... you won't be beating up the gearbox too much in the winter months... but change it to 75W80 GL5 as soon as the weather improves.
As far as I know, there's not many gearboxes that use bronze or copper in them.. probably the older the car, the more likely you are to find it. Renault boxes used it a lot (hence Total still makes a GL4 in this spec') and I know for sure my old Alfa 145 gearbox had bronze selector forks... so don't stick the GL5 in just anything, without checking first.
Ralf S.