Though it is now sung near the opening of the Western Mass, the Gloria's liturgical home before that adoption was morning prayer in the Greek speaking East, where a doxology opening with the angels' words from Luke 2:14 and continuing with additional praise not found in scripture developed as part of Orthros, the morning office, from at least the fourth century CE. Its move into the Western Mass happened gradually and, according to later medieval liturgical tradition, was for a considerable period restricted to bishops celebrating Mass, with ordinary priests only gradually gaining permission to include it on Sundays and feast days outside a bishop's presence, a restriction some medieval liturgical commentators explain, plausibly rather than certainly, as reflecting the hymn's exalted, quasi episcopal solemnity in its earlier use.
The text's non biblical portion, everything beyond the initial angelic greeting, was composed by an unknown author or authors working within the early Greek liturgical tradition, expanding a single scriptural line into a fuller doxological composition addressing the Father and the Lamb of God in language echoing but not quoting other biblical passages. Its ritual suppression during Advent and Lent, mentioned elsewhere in this atlas, is itself a comparatively later Western liturgical development, formalized as the wider medieval calendar of penitential seasons solidified, so both halves of the hymn's history, its Eastern morning office origin and its later Western seasonal suppression, developed independently of the biblical verse that opens it and reflect centuries of separate liturgical elaboration on either side of the Mediterranean.