There are losses in gearboxes that show up as heat and noise, they are not 100% efficient and this shows some of the reasons for those losses. If you are driving a 10MW alternator through a gearbox then even a small percentage of that power will cause a lot of gearbox heat that has got to be catered for. These large gearboxes also induce harmonics that have to be fully understood when they are being designed. You don’t want to hit a harmonic as you are winding up the mass of a 10MW alternator, either of the prime mover or the alternator. I suspect the noise from gearboxes contributes to the hum from wind turbines, they are big gearboxes, I had to do the lifting plan for one once.
Martin C