Leah, seeing that she had left bearing, while Bilhah, her sister's handmaid, bore Jacob two sons, concluded that it was Jacob's destiny to have four wives, her sister and herself, and their half-sisters Bilhah and Zilpah. Therefore she also gave him her handmaid to wife. Zilpah was the youngest of the four women. It was the custom of that time to give the older daughter the older handmaid, and the younger daughter the younger handmaid, as their dowry, when they got married.
Now, in order to make Jacob believe that his wife was the younger daughter he had served for, Laban had given Leah the younger handmaid as her marriage portion. This Zilpah was so young that her body betrayed no outward signs of pregnancy, and nothing was known of her condition until her son was born. Leah called the boy Gad, which means "fortune," or it may mean "the cutter," for from Gad was descended the prophet Elijah, who brings good fortune to Israel, and he also cuts down the heathen world.
Leah had other reasons, too, for choosing this name of double meaning. The tribe of Gad had the good fortune of entering into possession of its allotment in the Holy Land before any of the others, and, also, Gad the son of Jacob was born circumcised.