shooter vs gunman

shooter and gunman both are nouns.

Word	Noun	Adjective	Verb	Adverb
shooter	Yes	No	No	No
gunman	Yes	No	No	No

As nouns, gunman is a hyponym of shooter; that is, gunman is a word with a more specific, narrower meaning than shooter:

shooter: a person who shoots (usually with respect to their ability to shoot)
gunman: a person who shoots a gun (as regards their ability)

Other hyponyms of shooter include crack shot, marksman, sharpshooter, gun, trapshooter.

As nouns, shooter and gunman are synonyms defined as:

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

shooter (noun)	gunman (noun)
a large marble used for shooting in the game of marbles	a professional killer who uses a gun
a person who shoots (usually with respect to their ability to shoot)	a person who shoots a gun (as regards their ability)
a professional killer who uses a gun
(sports) a player who drives or kicks a ball at the goal (or a basketball player who shoots at the basket)
a gambler who throws dice in the game of craps