taxi vs gypsy cab

taxi and gypsy cab both are nouns.

Word	Noun	Adjective	Verb	Adverb
taxi	Yes	No	Yes	No
gypsy cab	Yes	No	No	No

As nouns, gypsy cab is a hyponym of taxi; that is, gypsy cab is a word with a more specific, narrower meaning than taxi:

taxi: a car driven by a person whose job is to take passengers where they want to go in exchange for money
gypsy cab: a taxicab that cruises for customers although it is licensed only to respond to calls

Other hyponyms of taxi include minicab.

taxi (noun)	gypsy cab (noun)
a car driven by a person whose job is to take passengers where they want to go in exchange for money	a taxicab that cruises for customers although it is licensed only to respond to calls

taxi (verb)	gypsy cab (verb)
ride in a taxicab
travel slowly