The gear itself doesn't wear at all from static force, since it's just pieces of metal meshing with each other but not moving. They can wear a bit from turning under power, though this is pretty negligible in vehicle transmissions that are working properly.

I think you're thinking of clutches. Manual transmission cars use clutches which physically slip to start from a stop. If you left one partially engaged while braked at a full stop, it would indeed wear very fast, so don't do that. Automatic transmission cars use torque converters, which as described in the other posts is a fluid coupling instead of a mechanical slippage, so it doesn't wear significantly from exerting force while the car is held at a stop by the brakes.