steering gear vs gear mechanism

steering gear and gear mechanism both are nouns.

Word	Noun	Adjective	Verb	Adverb
steering gear	Yes	No	No	No
gear mechanism	Yes	No	No	No

As nouns, gear mechanism is a hypernym of steering gear; that is, gear mechanism is a word with a broader meaning than steering gear:

steering gear: a gear that couples the steering wheel to the steering linkage of a motor vehicle
gear mechanism: a mechanism for transmitting motion for some specific purpose (as the steering gear of a vehicle)

Other hypernyms of steering gear include gear.

steering gear (noun)	gear mechanism (noun)
a gear that couples the steering wheel to the steering linkage of a motor vehicle	a mechanism for transmitting motion for some specific purpose (as the steering gear of a vehicle)