Source : Webster's Revised Unabridged Dictionary (1913)

Gainful \Gain"ful\, a.
   Profitable; advantageous; lucrative. ``A gainful
   speculation.'' --Macaulay. -- {Gain"ful*ly}, adv. --
   {Gain"ful*ness}, n.

Source : WordNet®

gainful
     adj : yielding a fair profit [syn: {paid}, {paying}]