The Collaborative International Dictionary of English v.0.48:

Gladness \Glad"ness\, n. [AS. gl[ae]dnes.]
   State or quality of being glad; pleasure; joyful
   satisfaction; cheerfulness.
   [1913 Webster]

         They . . . did eat their meat with gladness and
         singleness of heart.                     --Acts ii. 46.
   [1913 Webster]

   Note: Gladness is rarely or never equivalent to mirth,
         merriment, gayety, and triumph, and it usually
         expresses less than delight. It sometimes expresses
         great joy.
         [1913 Webster]

               The Jews had joy and gladness, a feast and a good
               day.                               --Esther viii.
                                                  17.
         [1913 Webster]

WordNet (r) 3.0 (2006):

gladness
    n 1: experiencing joy and pleasure [syn: gladness,
         gladfulness, gladsomeness]

Moby Thesaurus II by Grady Ward, 1.0:

55 Moby Thesaurus words for "gladness":
   beatification, beatitude, bewitchment, blessedness, bliss,
   blissfulness, blitheness, blithesomeness, brightness, cheer,
   cheerfulness, cheeriness, cheery vein, cloud nine, delectation,
   delight, ecstasy, ecstatics, elation, enchantment, eupeptic mien,
   exaltation, exhilaration, exuberance, felicity, gaiety, geniality,
   gladsomeness, glee, good cheer, happiness, heaven, high spirits,
   hopefulness, intoxication, joy, joyance, joyfulness, optimism,
   overhappiness, overjoyfulness, paradise, pleasantness, radiance,
   rapture, ravishment, rosy expectation, sanguine humor,
   sanguineness, seventh heaven, sunniness, sunshine, transport,
   unalloyed happiness, winsomeness