We don't know the history of this piece. It appears to be done on a thick, handmade paper with fiber or cloth content. See last two photos for close ups of the paper with a strong light behind it. Probably a cotton rag paper; it is not parchment or vellum. Not being experts we cannot date this other than saying its very old, certainly an antique, but can't say it's 1400 or 1500s although it could be.
Adapted from online sources: Gregorian chant is the central tradition of Western plainchant, a form of monophonic, unaccompanied sacred song in Latin (and occasionally Greek) of the Roman Catholic Church. Gregorian chant developed mainly in western and central Europe during the 9th and 10th centuries, with later additions and redactions.
Gregorian chant was traditionally sung by choirs of men and boys in churches, or by men and women of religious orders in their chapels. It is the music of the Roman Rite, performed in the Mass and the monastic Office. Although Gregorian chant supplanted or marginalized the other indigenous plainchant traditions of the Christian West to become the official music of the Christian liturgy, Ambrosian chant still continues in use in Milan, and there are musicologists exploring both that and the Mozarabic chant of Christian Spain. Although Gregorian chant is no longer obligatory, the Roman Catholic Church still officially considers it the music most suitable for worship.
Measures 15 1/2" x 10 1/4".