Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

gentleness
     n 1: the property possessed by a slope that is very gradual [syn:
           {gradualness}] [ant: {abruptness}]
     2: mildness of manner or disposition