low gear vs gear mechanism

low gear and gear mechanism both are nouns.

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

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

low gear: the lowest forward gear ratio in the gear box of a motor vehicle; used to start a car moving
gear mechanism: a mechanism for transmitting motion for some specific purpose (as the steering gear of a vehicle)

Other hypernyms of low gear include gear.

low gear (noun)	gear mechanism (noun)
the lowest forward gear ratio in the gear box of a motor vehicle; used to start a car moving	a mechanism for transmitting motion for some specific purpose (as the steering gear of a vehicle)