From Lux Occidentalis, by Fr. John Connely
Used by permission
The Liturgy of St. Peter (commonly known as the Liturgy of St. Gregory), is found, substantially as it has been used in the Latin Church until Vatican II (1969)1, in the Sacramentaries of St. Gregory , Gelasius  and St. Leo . The Roman Liturgy is attributed to St. Peter by ancient liturgical commentators, who founded their opinion chiefly upon a passage in an Epistle of Innocent [fifth century], to Decentius, Bishop of Eugubium. St. Gregory revised the variable parts of the liturgy, the Collects, Epistles, and Gospels; but the only change which he made in the Ordinary was by the addition of a few words which is noticed by the Venerable Bede [Hist. Eccl. Lib.2, c.I.].2
Since the time of St. Gregory the Roman Liturgy has been used over a large part of the Western Church, and, until 1969, was practically the only one allowed by Rome. From the Roman Liturgy in its primitive form were derived that used by the Churches of North-western Africa, and the Ambrosian Rite of the Church of Milan.