WordNet (r) 3.0 (2006):

gun down
    v 1: strike down or shoot down

Moby Thesaurus II by Grady Ward, 1.0:

50 Moby Thesaurus words for "gun down":
   blast, blot out, blow to pieces, blow up, brain, bring down,
   bump off, burn to death, croak, cut down, cut to pieces,
   deal a deathblow, disintegrate, do in, drop, erase, fell, fix,
   frag, get, give the business, give the quietus, hit, ice,
   incinerate, jugulate, lapidate, lay low, lay out, off, pistol,
   poleax, polish off, riddle, rub out, settle, shoot, shoot down,
   shoot to death, shotgun, silence, stab to death, stone,
   stone to death, strike dead, take care of, vaporize, waste,
   wipe out, zap