From WordNet (r) 2.0 (August 2003) [wn]:
gainful
adj : yielding a fair profit [syn: paid, paying]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
Gainful \Gain"ful\, a.
Profitable; advantageous; lucrative. "A gainful speculation."
--Macaulay. -- Gain"ful*ly, adv. -- Gain"ful*ness, n.
[1913 Webster]