To be gainfully employed means a person has consistent work and regular pay from an employer. This was a term created to track the success of higher education institutes.
Gainful employment is a legal term that was created to give meaning to the types of learning and training received from a higher education institute; it is directly applied to having a consistent job with regular pay. The government needed a way to track how well graduates were doing in the real world after they graduated from college. This information helps schools and training facilities qualify for federal funding. The term is mostly applied to graduates who accept a job after an internship or graduation.