Linked from
The 49 pages that link to Gregorian chant, each with the reason it gives.
MelodyBroader topic: Its unaccompanied melodic lines show how phrasing and text shape a tune.
Modal musicBroader topic: Its melodies were classified by medieval church modes.
Christian musicBroader topic: It became a defining sound of Western Christian liturgy.
Religious musicBroader topic: Its sung texts structure prayer and worship in the Roman liturgy.