WordNet (r) 3.0 (2006):

gangster
    n 1: a criminal who is a member of gang [syn: gangster,
         mobster]

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "gangster":
   Judas, Mafioso, bad person, bandit, betrayer, brigand, convict,
   criminal, crook, dacoit, deceiver, delinquent, desperado,
   desperate criminal, double-dealer, evildoer, felon, fugitive,
   gallows bird, gaolbird, goon, gunman, gunsel, gunslinger, hit man,
   hood, hoodlum, hooligan, jailbird, lawbreaker, malefactor,
   malevolent, malfeasant, malfeasor, misfeasor, mobsman, mobster,
   mugger, outlaw, public enemy, quisling, racketeer, robber, ruffian,
   scofflaw, sinner, skinhead, soldier, swindler, thief, thug,
   torpedo, tough, traitor, transgressor, two-timer, villain,
   worker of ill, wrongdoer