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