gunman vs gun for hire

gunman and gun for hire both are nouns.

Word	Noun	Adjective	Verb	Adverb
gunman	Yes	No	No	No
gun for hire	Yes	No	No	No

As nouns, gunman and gun for hire are synonyms defined as:

Other synonyms of gunman include gun, gunslinger, hired gun, hit man, hitman, shooter, torpedo, triggerman.

gunman (noun)	gun for hire (noun)
a professional killer who uses a gun	a professional killer who uses a gun
a person who shoots a gun (as regards their ability)