from the Synaxarion:
The Archangel Gabriel was chosen by the Lord to make the blessed announcement to the Virgin Mary about the Incarnation of the Son of God from Her, to the great rejoicing of all mankind. Therefore on the day after the feast of the Annunciation -- the day itself on which the All-Pure Virgin Herself is glorified, we give thanks to the Lord and we venerate His messenger Gabriel, who contributed to the mystery of our salvation.
The holy Archistrategos Gabriel acted in service to the Almighty God. He announced to Old Testament mankind about the future Incarnation of the Son of God; he inspired the Prophet Moses during the writing of the Pentateuch books of the Bible, he announced to the Prophet Daniel about the coming tribulations of the Hebrew People (Dan. 8: 16, 9: 21-24); he appeared to Righteous Anna with the news of the birth from her of the Ever-Blessed Virgin Mary. The holy Archangel Gabriel stayed constantly with the Holy Virgin Mary when She was a child in the Jerusalem Temple and afterwards watched over Her throughout all Her earthly life. He appeared to the Priest Zachariah, foretelling the birth of the Forerunner of the Lord, John the Baptist. The Lord dispatched him to Saint Joseph the Betrothed, where he appeared to him in a dream revealing to him the mystery of the Incarnation of the Son of God from the All-Pure Virgin Mary, and warned him of the wicked intentions of Herod, ordering him to flee into Egypt with the Divine-Infant and the Mother of God. When the Lord before His Passion prayed in the Garden of Gethsemane to the extent of sweating blood, according to Church tradition, to strengthen Him there was sent from Heaven the Archangel Gabriel, whose very name signifies "Strength of God" (Lk. 22: 43).
The Myrh-Bearing Women heard from the Archangel the joyous news about the Resurrection of Christ.
Mindful this day of the manifold appearances of the holy Archangel Gabriel and of his zealous fulfilling of the Divine Will, and confessing his intercession before the Lord for Christians, the Orthodox Church calls upon its children with faith and with fervor to have recourse in prayer to the great Angel.
Troparion of the Archangel Gabriel Tone 4
Supreme Leader of the heavenly Hosts,/ we implore thee that by thy prayers thou wilt encircle us/ unworthy as we are,/ with the protection of the wings of thine immaterial glory,/ and guard us who fall down before thee and fervently cry:/ Deliver us from dangers,/ for thou art the commander of the Powers above.
Kontakion of the Archangel Gabriel Tone 2
Supreme Leader of God's armies and minister of the divine glory,/ prince of the Bodiless Angels and guide of men,/ ask what is good for us and great mercy,/ as Supreme Leader of the Bodiless Hosts.
Kontakion of the Archangel Tone 8
O Archangel, thou art the glorious intercessor and minister/ of the Splendid, Holy, All-accomplishing, Ineffable and Awesome Trinity./ Pray that we may be delivered from all harm and torment,/ that we may cry to thee: Rejoice, O protector of thy servants.
May the Lord's faithful bodiless servant watch over you all today, my friends!