What vehicle and year?
GL-5 specific Diferential fluids should only be used in a differential.
Is this a Subaru driveline system? There are specific GL-5/GL-4 fluids for vehicles such as the Subaru because of a common sump system.
All manual transmission and differential fluids have deactivator chemistry.
Yes, I have an 07' 5sp NA Subaru Impreza, that I don't use a lot. Using Subaru's Extra-S gear oil in both the front transaxle and rear. Don't want to be using anything that could cause a problem. Rather get the Right Stuff if there is one. Seems hard to find out, and conflicting info. So far things seem good, and hope they stay that way.