The Collaborative International Dictionary of English v.0.48:

Gentleness \Gen"tle*ness\, n.
   The quality or state of being gentle, well-born, mild,
   benevolent, docile, etc.; gentility; softness of manners,
   disposition, etc.; mildness.
   [1913 Webster]

WordNet (r) 3.0 (2006):

gentleness
    n 1: the property possessed by a slope that is very gradual
         [syn: gradualness, gentleness] [ant: abruptness,
         precipitousness, steepness]
    2: acting in a manner that is gentle and mild and even-tempered;
       "his fingers have learned gentleness"; "suddenly her gigantic
       power melted into softness for the baby"; "even in the pulpit
       there are moments when mildness of manner is not enough"
       [syn: gentleness, softness, mildness]

Moby Thesaurus II by Grady Ward, 1.0:

153 Moby Thesaurus words for "gentleness":
   abnegation, abstinence, acceptance, affability, airiness,
   amiability, ascent, bleeding heart, bonhomie, bowels of compassion,
   breeding, bubbliness, buoyancy, calm, calmness, clemency,
   clementness, compassion, compassionateness, conservatism,
   constraint, continence, control, cool, cordiality, cultivation,
   culture, daintiness, decrescendo, delicacy, dimness, dispassion,
   dovelikeness, downiness, easiness, easygoingness, elegance,
   ethereality, evenness, faintness, featheriness, feebleness,
   flatness, floatability, flossiness, fluffiness, foaminess,
   fondness, forbearance, forbearing, frothiness, geniality,
   genteelness, gentility, gentlemanlikeness, gentlemanliness,
   golden mean, good breeding, good disposition, good humor,
   good nature, good temper, goodnaturedness, goodtemperedness,
   gossameriness, happy medium, humaneness, humanity, humility,
   impartiality, indistinctness, insolidity, judiciousness,
   juste-milieu, lack of weight, ladylikeness, lamblikeness, laxness,
   lenience, leniency, lenientness, lenity, levitation, levity,
   lightness, lowness, meden agan, meekness, mellowness, mercifulness,
   mercy, middle way, mildness, moderateness, moderation,
   moderationism, morbidezza, neutrality, nonresistiveness,
   nonrigidity, nonviolence, nothing in excess, pacifism, patience,
   peaceableness, pity, plushiness, polish, prudence, pulpiness,
   quietness, refinement, repose, restraint, ruthfulness, satininess,
   self-abnegation, self-control, self-denial, self-restraint,
   serenity, silkiness, sobriety, softheartedness, softness,
   sponginess, stability, steadiness, subaudibility, subduedness,
   sweetness, tameness, temperance, temperateness, tender feeling,
   tenderheartedness, tenderness, tolerance, tranquillity,
   unclearness, unexcessiveness, unextravagance, unextremeness,
   unheaviness, unsolidity, velvetiness, via media, volatility,
   warm heart, warmheartedness, weakness, weightlessness,
   yeastiness