WordNet (r) 3.0 (2006):

gunman
    n 1: a professional killer who uses a gun [syn: gunman,
         gunslinger, hired gun, gun, gun for hire,
         triggerman, hit man, hitman, torpedo, shooter]
    2: a person who shoots a gun (as regards their ability) [syn:
       gunman, gun]

Moby Thesaurus II by Grady Ward, 1.0:

84 Moby Thesaurus words for "gunman":
   Cain, Nimrod, adventurer, apache, archer, artilleryman, assassin,
   assassinator, bloodletter, bloodshedder, bowman, bravo, bucko,
   bully, bullyboy, burker, butcher, button man, cannibal, cannoneer,
   carabineer, condottiere, crack shot, cutthroat, dead shot, deadeye,
   desperado, devil, eradicator, executioner, exterminator,
   free lance, garroter, good shot, gorilla, gun, gunner, gunsel,
   gunslinger, hatchet man, head-hunter, hell-raiser, hellcat,
   hired gun, hired killer, hireling, hit man, homicidal maniac,
   homicide, hunter, killer, mad dog, man-eater, man-killer,
   manslayer, marksman, markswoman, massacrer, matador, mercenary,
   murderer, musketeer, pesticide, poison, poisoner,
   professional killer, rifleman, rough, rowdy, ruffian, sharpshooter,
   shooter, shot, slaughterer, slayer, sniper, soldier of fortune,
   strangler, targetshooter, thug, torpedo, toxophilite, trapshooter,
   trigger man