high gear vs gear mechanism

high gear and gear mechanism both are nouns.

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

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

high gear: a forward gear with a gear ratio that gives the greatest vehicle velocity for a given engine speed
gear mechanism: a mechanism for transmitting motion for some specific purpose (as the steering gear of a vehicle)

Other hypernyms of high gear include gear.

high gear (noun)	gear mechanism (noun)
a forward gear with a gear ratio that gives the greatest vehicle velocity for a given engine speed	a mechanism for transmitting motion for some specific purpose (as the steering gear of a vehicle)