Here "[Gad](rc://en/tw/dict/bible/names/gad)" stands for his descendants. AT: "The descendants of [Gad](rc://en/tw/dict/bible/names/gad) ... attack them, but they" (See: [[rc://en/ta/man/translate/figs-metonymy]])
Here "heels" stands for the raiders who are running away from the descendants of [Gad](rc://en/tw/dict/bible/names/gad). (See: [[rc://en/ta/man/translate/figs-synecdoche]])