The Collaborative International Dictionary of English v.0.48:

Gloom \Gloom\, v. i. [imp. & p. p. Gloomed; p. pr. & vb. n.
   Glooming.]
   1. To shine or appear obscurely or imperfectly; to glimmer.
      [1913 Webster]

   2. To become dark or dim; to be or appear dismal, gloomy, or
      sad; to come to the evening twilight.
      [1913 Webster]

            The black gibbet glooms beside the way. --Goldsmith.
      [1913 Webster]

            [This weary day] . . . at last I see it gloom.
                                                  --Spenser.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Gloom \Gloom\, v. t.
   1. To render gloomy or dark; to obscure; to darken.
      [1913 Webster]

            A bow window . . . gloomed with limes. --Walpole.
      [1913 Webster]

            A black yew gloomed the stagnant air. --Tennyson.
      [1913 Webster]

   2. To fill with gloom; to make sad, dismal, or sullen.
      [1913 Webster]

            Such a mood as that which lately gloomed
            Your fancy.                           --Tennison.
      [1913 Webster]

            What sorrows gloomed that parting day. --Goldsmith.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Gloom \Gloom\ (gl[=oo]m), n. [AS. gl[=o]m twilight, from the
   root of E. glow. See Glow, and cf. Glum, Gloam.]
   [1913 Webster]
   1. Partial or total darkness; thick shade; obscurity; as, the
      gloom of a forest, or of midnight.
      [1913 Webster]

   2. A shady, gloomy, or dark place or grove.
      [1913 Webster]

            Before a gloom of stubborn-shafted oaks. --Tennyson
                                                  .
      [1913 Webster]

   3. Cloudiness or heaviness of mind; melancholy; aspect of
      sorrow; low spirits; dullness.
      [1913 Webster]

            A sullen gloom and furious disorder prevailed by
            fits.                                 --Burke.
      [1913 Webster]

   4. In gunpowder manufacture, the drying oven.
      [1913 Webster]

   Syn: Darkness; dimness; obscurity; heaviness; dullness;
        depression; melancholy; dejection; sadness. See
        Darkness.
        [1913 Webster]

WordNet (r) 3.0 (2006):

gloom
    n 1: a state of partial or total darkness; "he struck a match to
         dispel the gloom" [syn: gloom, somberness,
         sombreness]
    2: a feeling of melancholy apprehension [syn: gloom,
       gloominess, somberness, sombreness]
    3: an atmosphere of depression and melancholy; "gloom pervaded
       the office" [syn: gloom, gloominess, glumness]

Moby Thesaurus II by Grady Ward, 1.0:

102 Moby Thesaurus words for "gloom":
   adumbrate, becloud, bedarken, bedim, begloom, black, black out,
   blacken, blackness, bleakness, block the light, blot out, blues,
   brood, brown, cast a shadow, cloud, cloud over, cloudiness, dark,
   dark shade, darken, darken over, darkle, darkness, dejection,
   depression, desolation, despair, despondency, dim, dim out,
   dimness, dismalness, doldrums, dolor, downheartedness, dreariness,
   dullness, dumps, dusk, eclipse, encloud, encompass with shadow,
   frown, gloam, gloominess, glower, gravity, grimace, grimness,
   grow dark, grow dim, knit the brow, look black, look sullen,
   low spirits, lower, lowering, make a lip, make a moue, melancholy,
   mere shadow, misery, mope, moroseness, murk, murkiness, obfuscate,
   obnubilate, obscure, obscurity, obumbrate, occult, occultate,
   overcast, overcloud, overshadow, penumbra, pout, sadness, scowl,
   shade, shadiness, shadow, shadowiness, shadows numberless,
   silhouette, skiagram, skiagraph, solemnity, somber, somberness,
   sombrousness, sorrow, umbra, umbrage, umbrageousness, unhappiness,
   wearifulness, wearisomeness, woe