third gear vs gear mechanism

third gear and gear mechanism both are nouns.

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

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

third gear: the third from the lowest forward ratio gear 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 third gear include gear.

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