A quick Google check is not conclusive. Some sources say not to use GL-5 in transmissions. If you want to be absolutely sure, purchase the fluid from a Dealer. It may cost a couple of bucks more, but you only need 3 quarts and you won't be doing it that often. Someone who has a manual transmission will probably come by to provide more pertinent information.
The GM part number is 89021806. A quick search online shows it still available as GL-3
How many miles is on yours ? I changed mine out at maybe 90-100k and it was still gold and i kinda regret changing it seeing it still so clean. But i put in ams oil in mine. Now ive got a noise from mine when i let off the gas at freeway speed. so if you can id buy factory fluid if you want to change it.