The adjective has 2 meanings:
Meaning #1:
of or relating to Pope Gregory I or to the plainsong chants of the Roman Catholic Church
Pertains to nouns: Gregory (meaning #3), Gregorian chant (meaning #1)
Meaning #2:
of or relating to Pope Gregory XIII or the calendar he introduced in 1582
Pertains to nouns: Gregory (meaning #2), Gregorian calendar (meaning #1)




