gearbox vs gear box

gearbox and gear box both are nouns.

Word	Noun	Adjective	Verb	Adverb
gearbox	Yes	No	No	No
gear box	Yes	No	No	No

As nouns, gearbox and gear box are synonyms defined as:

gearbox and gear box: the shell (metal casing) in which a train of gears is sealed

Other synonyms of gearbox include gear case.

gearbox (noun)	gear box (noun)
the shell (metal casing) in which a train of gears is sealed	the shell (metal casing) in which a train of gears is sealed