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 this icon, Gabriel wears white, the color of purity, and blue, the color of humanity and especially the Virgin Mary. He holds a bouquet of lilies, which are associated with the Annunciation to the Virgin Mary, an event Gabriel is most associated with.