Blessed be he who enlarges Gad

"May Yahweh bless Gad and give him much land to live in" or "People should say that Yahweh is good because he has given Gad much land to live in"

He will live there like a lioness, and he will tear off an arm or a head

This means the people of Gad are strong and secure, and they will defeat their enemies in war.