The Collaborative International Dictionary of English v.0.48:

Gigantic \Gi*gan"tic\, a. [L. gigas, -antis, giant. See
   Giant.]
   1. Of extraordinary size; like a giant.
      [1913 Webster]

   2. Such as a giant might use, make, or cause; immense;
      tremendous; extraordinarly; as, gigantic deeds; gigantic
      wickedness. --Milton.
      [1913 Webster]

            When descends on the Atlantic
            The gigantic
            Strom wind of the equinox.            --Longfellow.
      [1913 Webster]

WordNet (r) 3.0 (2006):

gigantic
    adj 1: so exceedingly large or extensive as to suggest a giant
           or mammoth; "a gigantic redwood"; "gigantic
           disappointment"; "a mammoth ship"; "a mammoth
           multinational corporation" [syn: gigantic, mammoth]

Moby Thesaurus II by Grady Ward, 1.0:

110 Moby Thesaurus words for "gigantic":
   Antaean, Atlantean, Briarean, Brobdingnagian, Cyclopean,
   Gargantuan, Herculean, Homeric, a bit much, abandoned, abysmal,
   amplitudinous, astronomic, astronomical, awesome, big, boundless,
   bulky, colossal, cosmic, cyclopean, egregious, elephantine,
   enormous, epic, exaggerated, excessive, exorbitant, extensive,
   extravagant, extreme, fabulous, fancy, galactic, gangling, gangly,
   gargantuan, giant, giantlike, gluttonous, heroic, high, huge,
   hulking, humongous, hyperbolic, hypertrophied, immeasurable,
   immense, immoderate, incontinent, infinite, inordinate,
   intemperate, jumbo, king-size, lank, lanky, large, leggy, lengthy,
   long, long-legged, mammoth, massive, massy, mighty, monster,
   monstrous, monumental, mountainous, out of bounds, out of sight,
   outrageous, outsize, overbig, overdeveloped, overgreat, overgrown,
   overlarge, overmuch, overweening, prodigious, profound, rangy,
   sizable, spacious, staggering, statuesque, steep, stiff, strapping,
   stupendous, tall, thumping, thundering, titanic, too much,
   towering, tremendous, unbridled, unconscionable, undue,
   unreasonable, unrestrained, vast, voluminous, walloping, weighty,
   whopping