Gad is one the tribes of Israel claiming descent from Jacob through Zilpah, the slave of first wife, Leah (see Genesis 35:26). Gad was one of the three tribes whose inheritance was on the eastern side of the Jordan River (most of Israel was on the western side; see Numbers 32:16–19). Because of this, the Bible describes the tribe of Gad (along with the tribe of Reuben and half of the tribe of Manasseh) as making a special covenant not to worship gods besides Jehovah and to always come to Israel’s aid in times of war (Joshua 22:21–29).
Jacob’s blessing of Gad in Genesis 49:19 is obscure, though it is based on wordplays on Gad’s name—of the six words in the Hebrew version of this verse, four are some form of a G/D root. Moses’s blessing in Deuteronomy 33:20–21 is clearer (and more positive). There Moses spoke of Gad’s military prowess and connection to law and justice.
Based on the Bible’s association of the tribes with the stones in the Aaronic high priest’s breastplate and on the description in Numbers of the tribes’ being arranged with the “ensign of his father’s house” (Numbers 2:2), later Jewish tradition identified the stones, flags, and symbols on those flags given to each tribe. Gad’s stone is the agate. The tribe’s flag is described as mix of black and white, although it is unclear whether this was an alternating pattern or some kind of gray. The image on the flag is a military camp, deriving from the description of Gad as a troop in Jacob’s blessing.[1]
[1][1] Numbers Rabbah 2:7, in Judah J. Slotki, Numbers Rabbah I (London, England: Soncino Press, 1939), 29–30.