There are GL5 oils that meet or exceed the GL4 spec. The issue is that some of the cheaper GL5 oils are reactive with zinc or copper containing alloys. So you would need to contact the MFG to confirm compatibility.
I used swepco's synthetic gear lube in my previous MT (finicky VW transaxle). Really improved cold weather shifting and synchro performance.
This subject has been discussed to death in the Benzworld Unimog forum. The consensus seems to be that GL4/5s are too “slippery” for synchos to work well. In the GL4 world you can go from 75W80 Redline to 75W85 and 75W90; lots of mfgs make the latter 2. I actually used Delvac 50 Transmission Fluid for 65k mi, it was a little slow shifting in the winter in Anchorage but fine otherwise; it’s a heavy 75W90. Now I am using the remaining Delvac 40% and some Redline 75W80 I had at home about 60%. Works better in the winter.
Mercedes is now recommending 75W80 GL4 in newer Unimog transmissions (with electro pneumatic shifting).
You have a 1017 or something like it with 5 or 6 spd with synchros. ?
I would use a 75W90 GL4.
I consulted Merex Germany, which is one of the decent Unimog parts supplier. I have been ordered parts from Merex since 2 years ago. About the transmission oil, my manual shows ExxonMobil Mobiltrans SHC50, for my U500. This trans oil has been opt out of market, so I took equivalent Mobiltrans SH50, however, this oil is definately not good for U500, after running only 1500miles, the gear shift gets problem. I topped off again the same oil, and again after 1500miles, the transmission gear shift got problem, so I consulted Merex, they recommended Castrol Z long life 75w80, said this is the proper one from the manual. I couldn't find it from the US market, so I use Redline 75w80 instead, so far, it feels perfectly nice.
For the differentials and portal axles, I use Mobil Delvac synthetic gear oil 75w90, after ten thousand miles, it looks good.