(g hard), in Jewish mythology, is the angel of death to the favoured people of God, the prince of fire and thunder, and the only angel that can speak Syriac and Chaldee. The Mahometans call him the chief of the four favoured angels, and the spirit of truth. In mediæval romance he is the second of the seven spirits that stand before the throne of God, and, as God's messenger, carries to heaven the prayers of men. (Jerusalem Delivered, book i.) The word means “power of God.” Milton makes him chief of the angelic guards placed over Paradise.
Betwixt these rocky pillars Gabriel sat, Chief of the angelic guards.
Paradise Lost, iv. 549-550.
Longfellow, in his Golden Legend, makes him the angel of the moon, and says he brings to man the gift of hope.
I am the angel of the moon ... Nearest the earth, it is my ray That best illumines the midnight way. I bring the gift of hope.
The Miracle Play, iii.
It was Gabriel who (we are told in the Koran) took Mahomet to heaven on Al-borak (q.v.), and revealed to him his “prophetic lore.” In the Old Testament Gabriel is said to have explained to Daniel certain visions; and in the New Testament it was Gabriel who announced to Zacharias the future birth of John the Baptist, and that afterwards appeared to Mary, the mother of Jesus. (Luke i. 26, etc.)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894