An angel is a messenger of God, and a being which has stood before God and praised Him from a time before human comprehension. We cannot fully grasp what an angel's exact nature is, but we can understand an angel's function on earth. In icons, angels—especially the high-ranking Archangels—are depicted in an earthly form. All the angels are dressed as humans, in clothing accented with gold, the color of holiness and Heaven.
In the Bible, Gabriel is a key liaison between God's work and human comprehension of it, so he is clothed in a deacon's vestments. His vestments are oftentimes green, a color which contrasts to red in its connection to peaceful renewal and the mediating power of the Holy Spirit through the sacraments. Gabriel also carries a reed, which shows that he bears messages in peace.
This icon of the Archangel Gabriel is from the Monastery of St. Catherine on Mount Sinai, Egypt. It was painted in the 13th century.