Numbers 32:27

But thy servants will pass over, every man armed for war, before the LORD to battle, as my lord saith.

Gad and Reuben pledged that after preparing homes for their families, every armed man among them would cross the Jordan. They would go before the Lord to battle together with the other tribes. They would continue fighting alongside their brethren until possession of the inheritance had been secured. Their settling on the east of the Jordan did not excuse them from that service. They still shared fully in the burdens and responsibilities belonging to all their brethren.