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.