The Collaborative International Dictionary of English v.0.48:

Gratuitous \Gra*tu"i*tous\a. [L. gratuitus, from gratus
   pleasing. See Grate, a., Gratis.]
   1. Given without an equivalent or recompense; conferred
      without valuable consideration; granted without pay, or
      without claim or merit; not required by justice.
      [1913 Webster]

            We mistake the gratuitous blessings of Heaven for
            the fruits of our own industry.       --L'Estrange.
      [1913 Webster]

   2. Not called for by the circumstances; without reason,
      cause, or proof; adopted or asserted without any good
      ground; as, a gratuitous assumption.
      [1913 Webster]

            Acts of gratuitous self-humiliation.  --De Quincye.
      -- Gra*tu"i*tous*ly, adv. -- Gra*tu"i*tous*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

gratuitous
    adj 1: without cause; "a gratuitous insult"
    2: costing nothing; "complimentary tickets"; "free admission"
       [syn: complimentary, costless, free, gratis(p),
       gratuitous]
    3: unnecessary and unwarranted; "a strikers' tent camp...was
       burned with needless loss of life" [syn: gratuitous,
       needless, uncalled-for]

Moby Thesaurus II by Grady Ward, 1.0:

180 Moby Thesaurus words for "gratuitous":
   God-given, accorded, aimless, allowed, amorphous, arbitrary,
   autonomous, baseless, bestowed, biggety, bluff, bottomless, brash,
   capricious, casual, charitable, cheeky, chutzpadik, cocky,
   complimentary, contemptuous, costless, crusty, de trop, derisive,
   desultory, disarticulated, discontinuous, discretional,
   discretionary, disjunct, disordered, dispensable, dispersed,
   disproportionate, disrespectful, elective, eleemosynary, erratic,
   excess, expendable, expenseless, expletive, facy, fitful, flip,
   flippant, for free, for love, for nothing, formless, free,
   free as air, free for nothing, free gratis, free of charge,
   free of cost, free will, freebie, fresh, frivolous, gally,
   giftlike, given, granted, gratis, groundless, haphazard,
   hit-or-miss, immethodical, impertinent, impudent, in excess,
   inchoate, incoherent, indefensible, independent, indiscriminate,
   irrational, irregular, malapert, meaningless, misshapen, needless,
   nervy, nonessential, nonmandatory, nonsymmetrical, nonsystematic,
   nonuniform, offered, on the house, optional, orderless, pert,
   planless, pleonastic, proffered, prolix, promiscuous, providential,
   random, reasonless, redundant, rude, sassy, saucy, self-acting,
   self-active, self-determined, self-determining, senseless,
   shapeless, smart, smart-alecky, smart-ass, spare, spasmodic,
   spontaneous, sporadic, straggling, straggly, supererogatory,
   superfluous, systemless, tautologic, tautologous, to spare,
   unarranged, unasked, unbesought, unbidden, unbought, uncalled-for,
   unclassified, uncoerced, uncompelled, undirected, unessential,
   unforced, unfounded, ungraded, ungrounded, uninfluenced, uninvited,
   unjoined, unjustifiable, unjustified, unlooked-for, unmethodical,
   unnecessary, unneeded, unordered, unorganized, unpaid-for,
   unpressured, unprompted, unrequested, unrequired, unsolicited,
   unsorted, unsought, unsupportable, unsymmetrical, unsystematic,
   untaxed, ununiform, unwarranted, unwelcome, vague, verbose,
   voluntary, volunteer, vouchsafed, wandering, wanton, willful,
   willing, wise-ass, without charge