second gear vs gear mechanism

second gear and gear mechanism both are nouns.

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

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

second gear: the gear that has the second lowest forward gear ratio in the gear box 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 second gear include gear.

second gear (noun)	gear mechanism (noun)
the gear that has the second lowest forward gear ratio in the gear box of a motor vehicle	a mechanism for transmitting motion for some specific purpose (as the steering gear of a vehicle)