The scene depicted in the above image is that of the annunciation to the shepherds.
The annunciation to the shepherds is an episode in the Nativity of Jesus described in the Bible in Luke 2, in which angels tell a group of shepherds about the birth of Jesus. It is a common subject of Christian art and of Christmas carols.
As described in verses 8–20 of the second chapter of the Gospel of Luke, shepherds were tending their flocks out in the countryside near Bethlehem, when they were terrified by the appearance of an angel. The angel explains that he has a message of good news for all people, namely that "Today in the town of David a Savior has been born to you; he is the Messiah, the Lord. This will be a sign to you: You will find a baby wrapped in cloths and lying in a manger."
After this, a great many more angels appear, praising God with the words "Glory to God in the highest heaven, and on earth peace to those on whom his favor rests." Deciding to do as the angel had said, the shepherds travel to Bethlehem, and find Mary and Joseph and the infant Jesus lying in the manger, just as they had been told. The adoration of the shepherds follows.
The Angel is holding the script (banner) of what the angels proclaimed to the shepherds:
Glory to God in the highest; and on earth peace to men of good will. (Luke 2:14)
The style of this picture is definitely from about the 15th century, as the image below is also of the same period:
This late 15th-century Flemish miniature shows the annunciation to the shepherds.
The words at the bottom of this image are from Psalm 69 as follows:
V. Deus, in adjutorium meum intende,
R. Domine, ad adiuvandum me festina
V. Make haste, O God, to deliver me;
R. Make haste to help me, O LORD."
These words form the introductory prayer to every Hour of the Roman, Ambrosian, and monastic Breviaries, except Matins and during the last three days of Holy Week, and in the Office of the Dead. While these are recited or sung, all present bless themselves with the sign of the cross.
Tradition says that Benedict of Nursia introduced this custom into the monastic Office, heavily influenced by the writings of Saint John Cassian. Saint Gregory the Great extended it to all the Roman churches. Saint John Cassian (Coll., X, 10) wrote that from the earliest Christian times the monks used this introduction very often, including outside of the liturgical prayers, as an invocation for every situation, for times of temptation, tiredness, and joy. - Deus, in adiutorium meum intende (Wikipedia)
It could be noted that the Gloria is not sung in Catholic Masses during Advent and is to be reserved for the joy of the Christmas Vigil Mass, Christmas Midnight Mass and the Christmas Mass during the Day!.
GIRM 53: The Gloria in excelsis (Glory to God in the highest) is a most ancient and venerable hymn by which the Church, gathered in the Holy Spirit, glorifies and entreats God the Father and the Lamb. The text of this hymn may not be replaced by any other. It is intoned by the Priest or, if appropriate, by a cantor or by the choir; but it is sung either by everyone together, or by the people alternately with the choir, or by the choir alone. If not sung, it is to be recited either by everybody together or by two choirs responding one to the other.
It is sung or said on Sundays outside Advent and Lent, and also on Solemnities and Feasts, and at particular celebrations of a more solemn character.