Gainfully employed is defined in SUPERANNUATION INDUSTRY (SUPERVISION) REGULATIONS 1994 - REG 1.03 .
Relevant definitions from regulation 1.03 include:
gainfully employed means employed or self-employed for gain or reward in any business, trade, profession, vocation, calling, occupation or employment.
full-time, in relation to being gainfully employed, means gainfully employed for at least 30 hours each week.
part-time, in relation to being gainfully employed, means gainfully employed for at least 10 hours, and less than 30 hours, each week.
In theory, working to supervise your SMSF can be classified as gainfully employed if you can prove it takes that much work in a month!
