Genesis 35:26
And the sons of Zilpah, Leah’s handmaid; Gad, and Asher: these are the sons of Jacob, which were born to him in Padan-aram. And the sons of Zilpah, Leah’s handmaid; Gad, and Asher: these are the sons of Jacob, which were born to him in Padan-aram.
The reckoning in Genesis 35:22-26 sets forth Jacob's sons as twelve in number. From those twelve sons sprang the twelve tribes of Israel. In that reckoning Gad and Asher stand as the sons given through Zilpah, Leah's handmaid.