Gearboxes are used to transfer Mechanical Power. Power must be input into this face and can output to any other face. When given a Redstone Signal the Gearbox will not transfer Mechanical Power.
The block can be rotated by shift-clicking it with an empty hand to make facing it easier.
If a Gearbox breaks from being overpowered, it can be repaired by clicking or crafting it with two Gears or