C. F. Keil and Franz Delitzsch on Genesis 49:19

Gen 49:19 “Gad - a press presses him, but he presses the heel.” The name Gad reminds the patriarch of גּוּד to press, and גּדוּד the pressing host, warlike host, which invades the land. The attacks of such hosts Gad will bravely withstand, and press their heel, i.e., put them to flight and bravely pursue them, not smite their rear-guard; for עקב does not signify the rear-guard even in Jos 8:13, but only the reserves (see my commentary on the passage). The blessing, which is formed from a triple alliteration of the name Gad, contains no such special allusions to historical events as to enable us to interpret it historically, although the account in 1Ch 5:18. proves that the Gadites displayed, wherever it was needed, the bravery promised them by Jacob. Compare with this 1Ch 12:8-15, where the Gadites who come to David are compared to lions, and their swiftness to that of roes.