The Collaborative International Dictionary of English v.0.48:

Gaucho \Gau"cho\ (gou"ch[-o]), n., pl. Gauchos (gou"ch[-o]z)
   [Sp.]
   One of the native inhabitants of the South American pampas,
   of Spanish-American descent. They live mostly by rearing
   cattle. Hence, a South American cowboy, especially on the
   pampas.
   [1913 Webster]

   2. A member of an Indian population, somewhat affected by
      Spanish blood, in the archipelagoes off the Chilean coast.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

gaucho
    n 1: a cowboy of the South American pampas

Moby Thesaurus II by Grady Ward, 1.0:

50 Moby Thesaurus words for "gaucho":
   broncobuster, buckaroo, caballero, cattleman, cavalier, cavalryman,
   chevalier, circus rider, cowboy, cowgirl, cowhand, cowherd, cowman,
   cowpuncher, drover, equestrian, equestrienne, goatherd, gooseboy,
   gooseherd, herder, herdsman, hogherd, horse soldier,
   horse wrangler, horseback rider, horsebacker, horseherd, horseman,
   horsewoman, jockey, knight, mounted policeman, pigman, postboy,
   postilion, puncher, rider, roughrider, sheepherder, sheepman,
   shepherd, shepherdess, steeplechaser, swanherd, swineherd,
   trick rider, vaquero, waddy, wrangler