IMO which GL oil to use is no different than a motor oil discussion. It's basically a religious argument based on empirical evidence mostly from reading labels, manufacturer's claims, and seat of the pants experience. Bottom line use what you're most comfortable with. I agree use the correct weight though.
Having said that if you ask why GL-5 and other more modern lubes were developed it wasn't because the industry said we need a better lubricant for a 1930s, 40s or 50s 3 speed transmission. It was because lubrication requirements of the machines changed as technology advanced.
A more modern transmission has different operating parameters. It operates under multiple times more horsepower, much higher rpms, tighter machined tolerances, more complex design, extreme heat cycles, and so on.
Is a GL-5 backwards compatible with a transmission that originally used GL-1? Probably. Will that transmission benefit from GL-5, especially in the long term? Who knows. Most of us don't use these cars for daily drivers, so we're not subjecting the transmissions to extreme use. Given the relatively low operating parameters of our transmissions, who knows if the superiority of a GL-5 will have any benefits in an active way? Most likely condensation from sitting around most of the time will do more long term damage than your choice of lube.
Just do the proper preventative maintenance and the transmission will probably out last you regardless of your lubrication religion.