reverse gear vs gear mechanism

reverse gear and gear mechanism both are nouns.

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

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

reverse gear: the gears by which the motion of a machine can be reversed
gear mechanism: a mechanism for transmitting motion for some specific purpose (as the steering gear of a vehicle)

Other hypernyms of reverse gear include gear.

reverse gear (noun)	gear mechanism (noun)
the gears by which the motion of a machine can be reversed	a mechanism for transmitting motion for some specific purpose (as the steering gear of a vehicle)