Charles Ellicott Commentary Genesis 49:19

"Gad, a troop shall press upon him; But he shall press upon their heel." — Genesis 49:19 (ASV)

Gad. —The word Gad, as we have seen (Genesis 30:11), means good fortune, but Jacob connects it with the root gâdad, “to gather in troops.” Thus, then, A troop or throng of plunderers shall throng upon him, but he shall throng upon their heel. Settling upon the east of the Jordan he shall be exposed to many a sudden incursion of plunderers, but, though ever unready, he shall gather his forces and repel them, and follow with avenging energy upon their rear.