Jamieson, Fausset and Brown on Deuteronomy 33:20-21

20-21. of Gad he said--Its possessions were larger than they would have been had they lain west of Jordan; and this tribe had the honor of being settled by Moses himself in the first portion of land conquered. In the forest region, south of the Jabbok, "he dwelt as a lion" (compare Ge 30:11; 49:19). Notwithstanding, they faithfully kept their engagement to join the "heads of the people" [De 33:21] in the invasion of Canaan.