Saint Gabriel is one of the known Archangels which means he is one of the Chief Angels.
Hebrew: גַּבְרִיאֵל Gabriel (God is my strength)
Saint Gabriel is first mentioned in the book of Daniel, where he appears to Daniel to explain his visits:
15 While I, Daniel, was watching the vision and trying to understand it, there before me stood one who looked like a man. 16 And I heard a man’s voice from the Ulai calling, “Gabriel, tell this man the meaning of the vision.”
17 As he came near the place where I was standing, I was terrified and fell prostrate. “Son of man,” he said to me, “understand that the vision concerns the time of the end.”
18 While he was speaking to me, I was in a deep sleep, with my face to the ground. Then he touched me and raised me to my feet.
19 He said: “I am going to tell you what will happen later in the time of wrath, because the vision concerns the appointed time of the end.20 The two-horned ram that you saw represents the kings of Media and Persia. 21 The shaggy goat is the king of Greece, and the large horn between its eyes is the first king. 22 The four horns that replaced the one that was broken off represent four kingdoms that will emerge from his nation but will not have the same power.
23 “In the latter part of their reign, when rebels have become completely wicked, a fierce-looking king, a master of intrigue, will arise. 24 He will become very strong, but not by his own power. He will cause astounding devastation and will succeed in whatever he does. He will destroy those who are mighty, the holy people. 25 He will cause deceit to prosper, and he will consider himself superior. When they feel secure, he will destroy many and take his stand against the Prince of princes. Yet he will be destroyed, but not by human power.
26 “The vision of the evenings and mornings that has been given you is true, but seal up the vision, for it concerns the distant future.”
And again in Daniel 9:21
21 while I was still in prayer, Gabriel, the man I had seen in the earlier vision, came to me in swift flight about the time of the evening sacrifice. 22 He instructed me and said to me, “Daniel, I have now come to give you insight and understanding. 23 As soon as you began to pray, a word went out, which I have come to tell you, for you are highly esteemed. Therefore, consider the word and understand the vision:
24 “Seventy ‘sevens’ are decreed for your people and your holy city to finish transgression, to put an end to sin, to atone for wickedness, to bring in everlasting righteousness, to seal up vision and prophecy and to anoint the Most Holy Place.
25 “Know and understand this: From the time the word goes out to restore and rebuild Jerusalem until the Anointed One, the ruler, comes, there will be seven ‘sevens,’ and sixty-two ‘sevens.’ It will be rebuilt with streets and a trench, but in times of trouble. 26 After the sixty-two ‘sevens,’ the Anointed One will be put to death and will have nothing. The people of the ruler who will come will destroy the city and the sanctuary. The end will come like a flood: War will continue until the end, and desolations have been decreed. 27 He will confirm a covenant with many for one ‘seven.’ In the middle of the ‘seven’ he will put an end to sacrifice and offering. And at the temple he will set up an abomination that causes desolation, until the end that is decreed is poured out on him.”
He is then shown in the New Testament, appearing to the father of John the Baptist. He appears to Zacharias, a priest of the course of Abia. This is recorded in Luke 1:10-20:
10 And the whole multitude of the people were praying without at the time of incense.
11 And there appeared unto him an angel of the Lord standing on the right side of the altar of incense.
12 And when Zacharias saw him, he was troubled, and fear fell upon him.
13 But the angel said unto him, Fear not, Zacharias: for thy prayer is heard; and thy wife Elisabeth shall bear thee a son, and thou shalt call his name John.
14 And thou shalt have joy and gladness; and many shall rejoice at his birth.
15 For he shall be great in the sight of the Lord, and shall drink neither wine nor strong drink; and he shall be filled with the Holy Ghost, even from his mother's womb.
16 And many of the children of Israel shall he turn to the Lord their God.
17 And he shall go before him in the spirit and power of Elias, to turn the hearts of the fathers to the children, and the disobedient to the wisdom of the just; to make ready a people prepared for the Lord.
18 And Zacharias said unto the angel, Whereby shall I know this? for I am an old man, and my wife well stricken in years.
19 And the angel answering said unto him, I am Gabriel, that stand in the presence of God; and am sent to speak unto thee, and to shew thee these glad tidings.
20 And, behold, thou shalt be dumb [deaf], and not able to speak, until the day that these things shall be performed, because thou believest not my words, which shall be fulfilled in their season.
After Elizabeth was six months pregnant, Gabriel appears for a second time in New Testament, namely in Luke 1:26-38 to the Virgin Mary, announcing the coming of Jesus.
26 And in the sixth month the angel Gabriel was sent from God unto a city of Galilee, named Nazareth,
27 To a virgin espoused to a man whose name was Joseph, of the house of David; and the virgin's name was Mary.
28 And the angel came in unto her, and said, Hail, thou that art highly favoured, the Lord is with thee: blessed art thou among women.
29 And when she saw him, she was troubled at his saying, and cast in her mind what manner of salutation this should be.
30 And the angel said unto her, Fear not, Mary: for thou hast found favour with God.
31 And, behold, thou shalt conceive in thy womb, and bring forth a son, and shalt call his name JESUS.
32 He shall be great, and shall be called the Son of the Highest: and the Lord God shall give unto him the throne of his father David:
33 And he shall reign over the house of Jacob for ever; and of his kingdom there shall be no end.
34 Then said Mary unto the angel, How shall this be, seeing I know not a man?
35 And the angel answered and said unto her, The Holy Ghost shall come upon thee, and the power of the Highest shall overshadow thee: therefore also that holy thing which shall be born of thee shall be called the Son of God.
36 And, behold, thy cousin Elisabeth, she hath also conceived a son in her old age: and this is the sixth month with her, who was called barren.
37 For with God nothing shall be impossible.
38 And Mary said, Behold the handmaid of the Lord; be it unto me according to thy word. And the angel departed from her.
One of the earliest sources for Saint Gabriel's feast day being celebrated on March 18th was first published in 1608 : Flos sanctorum: historia general de la vida y hechos de Jesu-Christo...y de los santos de que reza y haze fiesta la Iglesia Catholica by Alonso de Villegas. Pope Benedict XV included the feast day in the General Roman Calendar in 1921, set for March 24th. This date was moved in 1969 to the date of September 29th for celebration together with Saint Michael and Saint Raphael.
The Eastern Orthodox church celebrates July 13th as the Synaxis of the Archangel Gabriel, a day in which all of the miracles attributed to Gabriel throughout history are celebrated. This feast was first established during the reign of Emporer Basil II in the 9th century at Mount Athos when the Archangel appeared to a monk, using his finger on a stone tablet to write the hymn to the Theotokos:
Ἄξιόν ἐστιν ὡς ἀληθῶς,
μακαρίζειν σε τὴν Θεοτόκον,
τὴν ἀειμακάριστον καὶ παναμώμητον
καὶ μητέρα τοῦ Θεοῦ ἡμῶν.
Τὴν τιμιωτέραν τῶν Χερουβεὶμ
καὶ ἐνδοξοτέραν ἀσυγκρίτως τῶν Σεραφείμ,
τὴν ἀδιαφθόρως Θεὸν Λόγον τεκοῦσαν,
τὴν ὄντως Θεοτόκον,
It is truly right to bless thee, O Theotokos,
thou the ever blessed, and most pure, and the Mother of our God.
Thou the more honorable than the cherubim,
and beyond compare more glorious than the seraphim,
who without corruption gavest birth to God the Word,
thou the true Theotokos, we magnify thee.
The Coptic Orthodox Church celebrates his feast day on 13 and 26 Paoni and 22 Koiak.
The Ethiopian Church celebrates his feast day on December 28th, with a massive pilgrimage to Saint Gabriel church in Kulubi.
Among many others, Saint Gabriel is the patron Saint of telecommunication workers, messengers, postal workers, diplomats and ambassadors.