Gear box and Reducer

Gear box noun - The shell (metal casing) in which a train of gears is sealed.

Reducer and gear box are semantically related in gear mechanism topic. In some cases you can use "Reducer" instead a noun phrase "Gear box", when it comes to topics like reduction gear.

Reducer noun - A substance capable of bringing about the reduction of another substance as it itself is oxidized; used in photography to lessen the density of a negative or print by oxidizing some of the loose silver.

Gear box and reducer are semantically related in gear mechanism topic. You can use "Gear box" instead a noun "Reducer", if it concerns topics such as reduction gear.