The Collaborative International Dictionary of English v.0.48:

Gladden \Glad"den\, v. i.
   To be or become glad; to rejoice.
   [1913 Webster]

         The vast Pacific gladdens with the freight.
                                                  --Wordsworth.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Gladden \Glad"den\, v. t. [imp. & p. p. Gladdened; p. pr. &
   vb. n. Gladdening.] [See Glad, v. t.]
   To make glad; to cheer; to please; to gratify; to rejoice; to
   exhilarate.
   [1913 Webster]

         A secret pleasure gladdened all that saw him.
                                                  --Addison.
   [1913 Webster]

WordNet (r) 3.0 (2006):

gladden
    v 1: make glad or happy [syn: gladden, joy] [ant: sadden]
    2: become glad or happy [ant: sadden]

Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "gladden":
   animate, beatify, bless, boost, brighten, buoy up, cheer, comfort,
   console, delectate, delight, elate, encourage, enliven, exhilarate,
   give a lift, gratify, happify, hearten, inspire, inspirit,
   invigorate, liven, make happy, pick up, pleasure, quicken, rejoice,
   rejoice the heart, solace, vitalize, vivify