The festival of St. Gabriel was appointed for this day in the Roman Missal by Pope Benedict XV. It is placed close to the festival of the Annunciation of the Blessed Virgin to whom he announced that she had been chosen to become the Mother of God. In the Old Testament, he is mentioned in the Book of Daniel, and he was the Archangel who appeared to the High Priest, Zachary, the father of John the Baptist.
(Source: Fr. Lasance, The New Roman Missal)