I’ve seen fiberglass reinforced nylon gears used with success in commercial injection molded gearboxes. It can definitely help by increasing gear stiffness.

However 3D printing with fiberglass causes rapid nozzle wear, and recently people are saying that fiberglass reinforced 3D printing filaments represent a human hazard.

Since I’ve managed to get my gears to work without reinforcement, I’ve not tried it.

One thing I do with printed gears is make the gear teeth much larger than would be strictly needed in an injection molded solution. This lets me deal with reduced material performance compared to injection molding.