The Collaborative International Dictionary of English v.0.48:

Gloominess \Gloom"i*ness\, n.
   State of being gloomy. --Addison.
   [1913 Webster]

WordNet (r) 3.0 (2006):

gloominess
    n 1: an atmosphere of depression and melancholy; "gloom pervaded
         the office" [syn: gloom, gloominess, glumness]
    2: a feeling of melancholy apprehension [syn: gloom,
       gloominess, somberness, sombreness]
    3: the quality of excessive mournfulness and uncheerfulness
       [syn: gloominess, lugubriousness, sadness]

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "gloominess":
   bleakness, cynicism, darkness, defeatism, dismalness, dreariness,
   gloom, gloomy outlook, gravity, grimness, lower, lowering, malism,
   negativism, nihilism, pessimism, retreatism, solemnity, somber,
   somberness, sombrousness, uncheerfulness, wearifulness,
   wearisomeness