The term 'Gregorian, for one' is referring to the Gregorian CHANT, which is a form of monophonic, unaccompanied sacred song of the Roman Catholic Church. It is named after Pope Gregory I, who is credited with its formalization in the 6th century. The CHANT is sung by a soloist or a choir, without any musical accompaniment. It is still used today in religious services and is considered to be one of the oldest forms of music.