The archangel Gabriel plays an extremely important role in all three of the major Abrahamic religions. Throughout scripture he often serves as God's messenger. He is first mentioned in the Old Testament Book of Daniel, where he appears as a man so that he can assist the prophet in deciphering several of his visions. In the Christian New Testament book of Luke, Gabriel is the angel that foretells the birth of both John the Baptist, as well as the birth of Jesus Christ. In Islam, he's arguably the most important heavenly angel, having been the medium by which God revealed the Qur'an to Muhammad. He is also the angel that revealed the duties to be carried out by each of Islam's most important prophets. Some scholars would even go so far as to say that Gabriel is the most important archangel out to the renown seven, however the consensus seems to be that both Michael and Gabriel have equally important roles to play in both heaven and on earth.
In Judaism, the archangel Gabriel is an extremely important figure. The world is first introduced to the angel, while the prophet Daniel is deciphering the meaning of several visions that he's had. After struggling to come to terms with these visions, Gabriel appears to Daniel in the form of a man, and then explains to the prophet what these revelations mean. In the Book of Daniel Chapter 8 verses 15 through19, the scripture states that Daniel heard God's mighty voice over the banks of the Ulai, and this loud voice commanded the archangel Gabriel to make Daniel understand the visions that he had just had. As Gabriel approached Daniel in human form, Daniel immediately falls on his face in humility. The angel then gently forces Daniel to sit upright, and tells him that his visions are of the end of days. These passages in the Christian Bible being directly linked with the fate of Israel, sealed Gabriel's destiny as a very important Biblical figure.
Gabriel also has an important role to play in Christianity. He's first mentioned in the New Testament in the Book of Luke Chapter 1 verses 10 through 20, where he appears to John's parents. In this set of scripture, it states that a large gathering of people were praying, and all of the sudden an archangel appeared over an incense altar. Zacharias being extremely fearful of what he was witnessing, stood trembling before the Lord's angel. Gabriel then said to Zacharias, have no fear, for the Lord has heard your prayer. Further into the Book of Luke however, Gabriel's most important role of all is fulfilled. In Chapter 1 verses 26 through 37, Gabriel appears to the Virgin Mary, and tells her that she is blessed and favored by God. He then tells Mary that she will conceive a son, and that he shall be called Jesus. Out of all of Gabriel's activities throughout the Old and New Testament, this one task is obviously the most important in his role as an angel.
Within Islam, the archangel Gabriel is also extremely important, as he was who God sent to reveal the Qur'an to the prophet Muhammad. Beyond that huge task, Gabriel is also the angel that accompanied Muhammad into heaven. Muslims honor Gabriel during the night of Laylat al-Qadr, when they believe the angel visits the earth. This annual visit by Gabriel marks the night of the calendar year when the Qur'an was first revealed to the prophet Muhammad. The archangel Gabriel is honored by all Muslims in the exact same way that they would honor any of the prophets. When a Muslim refers to Gabriel, they always follow his name with "peace be upon him". Muslims also believe that it was Gabriel that informed the Virgin Mary that she would give birth to Jesus, whom they regard as a great and most holy prophet. The undeviating role Gabriel plays throughout both Islam and Christianity is quite amazing to most religious scholars.