The Holy Archangel Gabriel has been called the leader of the heavenly hosts; he is associated with numerous happenings in Holy Scripture, particularly his revelation to the Theotokos that she would bear Christ. Thus his primary role has been called one of announcing the salvation of mankind. The Church celebrates the Synaxis of the Archangel Gabriel on March 26, the day following the Feast of the Annunciation, and again on July 13; he is also commemorated together with all the Archangels on their Synaxis date, November 8.
The name Gabriel comes from the Hebrew meaning "Man of God." It has alternately been translated "God is mighty" or "the strength/power of God." The Prologue from Ohrid explains his name this way: "Man-God. The Holy Fathers, in speaking about the Annunciation, interpret that an archangel with such a name was sent to signify who and what He would be like, who must be born of the All-Pure One. Therefore, He will be Man-God, mighty and powerful God."
Gabriel and Michael are the archangels who figure most prominently in the Bible, though it could be argued that Gabriel's role is the better developed. In the Old Testament, he is only mentioned by name in two visions of the Prophet Daniel (see Daniel 8 and 9). Here he explains to Daniel the future of Israel. Holy Tradition also credits Gabriel with inspiring the Prophet Moses to write either Genesis or the entire Pentateuch. Later Jewish Rabbinical literature states that he was the angel who taught Joseph the 70 languages needed to rule in Egypt, but this is not in the Genesis account.
Often Gabriel is also recognized as the angel who announced the birth of the Theotokos to her parents Joachim and Anna and who came to Joseph the Betrothed in a dream, telling him that Mary's pregnancy was indeed miraculous and that he should protect and care for her. He then appeared to the shepherds near Bethlehem, telling them of the Nativity. Thus he was the key figure in revealing to humanity the Incarnation of our Lord Jesus Christ. He will also announce the Second Coming of the Lord by blowing a trumpet.
Finally, Gabriel was present during the Passion and Resurrection of the Lord. He is identified as the mysterious "young man, wearing nothing but a linen garment ... following Jesus" who fled naked after he was seized during Christ's arrest in Gethsemene (Mark 14:51-2). Most importantly, it was Gabriel who announced Christ's Resurrection to the Myrrh-bearing Women outside the tomb.
Gabriel in Iconography
Because the Angels are incorporeal beings, though they nevertheless take on human form when appearing to mankind, it can be difficult to differentiate one from another in icons.
However, Gabriel is usually portrayed with certain distinguishing characteristics. He typically wears blue or white garments; he holds either a lily (representing the Theotokos), a trumpet, a shining lantern, a branch from Paradise presented to him by the Theotokos, or a spear in his right hand and often a mirror—made of jasper and with a Χ (the first letter of Christ (Χριστος) in Greek)—in his left hand.
He should not be confused with the Archangel Michael, who carries a sword, shield, date-tree branch, and in the other hand a spear, white banner (possibly with scarlet cross) and tends to wear red. Michael's specific mission is to suppress enemies of the true Church (hence the military theme), while Gabriel's is to announce mankind's salvation.
Material reprinted from https://orthodoxwiki.org/Archangel_Gabriel
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The reason why Gabriel is most celebrated, though, is his role in the Annunciation and other events in New Testament times attributed to him by Tradition (although his name may not be mentioned explicitly in the text). Starting in Luke 1, Gabriel first appears to Zachariah, the father of John the Baptist. Zachariah initially refuses to believe that his barren wife, Elizabeth, and he will have a child in their old age. This is the moment in which Gabriel says, "I am Gabriel. I stand before God, and I have been sent to speak to you and to tell you this this good news" (Luke 1:19). He then strikes Zachariah mute until the birth of his son because of his disbelief.