gunman vs hired gun

gunman and hired gun both are nouns.

Word	Noun	Adjective	Verb	Adverb
gunman	Yes	No	No	No
hired gun	Yes	No	No	No

As nouns, gunman and hired gun are synonyms defined as:

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

gunman (noun)	hired gun (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)