girl vs gal

girl and gal both are nouns.

Word	Noun	Adjective	Verb	Adverb
girl	Yes	No	No	No
gal	Yes	No	No	No

As nouns, gal is a hyponym of girl; that is, gal is a word with a more specific, narrower meaning than girl:

girl (noun)	gal (noun)
a young female	alliterative term for girl (or woman)
a girl or young woman with whom a man is romantically involved	United States liquid unit equal to 4 quarts or 3.785 liters
a youthful female person	a unit of gravitational acceleration equal to one centimeter per second per second (named after Galileo)
a friendly informal reference to a grown woman
a female human offspring