The Collaborative International Dictionary of English v.0.48:

Gainful \Gain"ful\, a.
   Profitable; advantageous; lucrative. "A gainful speculation."
   --Macaulay. -- Gain"ful*ly, adv. -- Gain"ful*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

gainful
    adj 1: yielding a fair profit [syn: gainful, paid, paying]

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "gainful":
   advantageous, banausic, beneficial, breadwinning, fat, fruitful,
   generous, good, lucrative, lush, moneymaking, of value, paying,
   productive, profitable, remunerative, rewarding, rich, satisfying,
   substantial, useful, valuable, well, well-paying, well-spent,
   worthwhile, yielding a return