What does Numbers 32:29 mean?

He said, 'If the men of Gad and Reuben cross the Jordan with you, armed for battle before the Lord, and you conquer the land, then give them the land of Gilead as their property.' Numbers 32:29 - Modern Text Bible

(Moses tells the leaders that if Gad and Reuben keep their promise, they get the land they asked for.)