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.
This icon is from the deësis (supplication) row of a 17th-century Russian iconostasis.