Sforno on Genesis 49:19/ספורנו על בראשית מ״ט:י״ט

1גד גדוד יגודנו, after having said that Dan fights like a snake felling horse and rider, Yaakov now contrasts Gad’s mode of operation with that of Dan by saying that the Gadites act in groups. Moses also repeats this theme in Deut. 33,20 when he said וטרף זרועו קדקוד, “he tore off arm and scalp.”

2והוא יגוד עקב, when he fells horse and rider he will cut off their heels.. This is because they will try to flee from his pursuit. Compare Exodus 23,27 ונתתי את כל אויביך אליך עורף, “I will make all your enemies flee from before you.”