Numbers 32:29
And Moses said unto them, If the children of Gad and the children of Reuben will pass with you over Jordan, every man armed to battle, before the LORD, and the land shall be subdued before you; then ye shall give them the land of Gilead for a possession:
Possession of Gilead was granted to Gad and Reuben only on condition of armed service alongside the other tribes beyond the Jordan. They were required to cross over equipped for battle and take part in subduing the land before the Lord. Their own inheritance was to be received only after they had helped their brethren secure theirs.