A "gypsy cab" is an unlicensed taxi. The use became popular in the 1960s.
23 November 1964, New York Times, pg. 39:
Gypsy Cabs Cruise City - and Thrive - Illegally
The gypsy drivers are permitted by law to respond only to telephoned or otherwise pre-arranged appointments. But in practice they stop when hailed and thrive in competition with the regular, city-licensed cab drivers, who are bitterly resentful of the gypsies.