The Collaborative International Dictionary of English v.0.48:

Gullible \Gul"li*ble\, a.
   Easily gulled; that may be duped. -- Gul"li*bii`i*ty, n.
   --Burke.
   [1913 Webster]

WordNet (r) 3.0 (2006):

gullible
    adj 1: naive and easily deceived or tricked; "at that early age
           she had been gullible and in love" [syn: fleeceable,
           green, gullible]
    2: easily tricked because of being too trusting; "gullible
       tourists taken in by the shell game"

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "gullible":
   artless, befoolable, born yesterday, credulous, cullible,
   deceivable, deludable, dupable, easy, exploitable, foolable, green,
   hoaxable, hoodwinkable, humbugable, immature, inexperienced,
   ingenuous, innocent, naive, persuadable, seduceable, simple, soft,
   susceptible, unsophisticated, unsuspecting, unsuspicious, unwary,
   victimizable, wide-eyed