It's very common to use GL-5 in manual gearboxes. Been done for decades. Not for every trans, but lots.
If I paid someone to 'build' a car for me, which I wouldn't, I'd beat them to death with a lead pipe if they ran the speedo cable under the carpet. Do it right, or change to a GPD speedo, or whatever. But running it under the carpet doesn't begin to approach proper workmanship.
