The Collaborative International Dictionary of English v.0.48:

Gargantuan \Gar*gan"tu*an\ (?; 135), a. [From Gargantua, an
   allegorical hero of Rabelais.]
   Characteristic of Gargantua, a gigantic, wonderful personage;
   enormous; prodigious; inordinate.
   [1913 Webster]

WordNet (r) 3.0 (2006):

gargantuan
    adj 1: of great mass; huge and bulky; "a jumbo jet"; "jumbo
           shrimp" [syn: elephantine, gargantuan, giant,
           jumbo]

Moby Thesaurus II by Grady Ward, 1.0:

51 Moby Thesaurus words for "Gargantuan":
   Atlantean, Brobdingnagian, Cyclopean, Herculean, Homeric, abysmal,
   amplitudinous, astronomic, astronomical, awesome, boundless, bulky,
   colossal, cosmic, elephantine, enormous, epic, extensive, galactic,
   giant, giantlike, gigantic, heroic, huge, immeasurable, immense,
   infinite, jumbo, king-size, large, mammoth, massive, massy, mighty,
   monster, monstrous, monumental, mountainous, outsize, overgrown,
   prodigious, profound, sizable, spacious, stupendous, titanic,
   towering, tremendous, vast, voluminous, weighty