John Gill Commentary Numbers 32:29

"And Moses said unto them, If the children of Gad and the children of Reuben will pass with you over the Jordan, every man that is armed to battle, before Jehovah, and the land shall be subdued before you; then ye shall give them the land of Gilead for a possession:" — Numbers 32:29 (ASV)

And Moses said unto them To Eleazar, Joshua, and the princes of the congregation:

if the children of Gad and the children of Reuben will pass with you over Jordan, every man armed to battle before the Lord ; as they have promised they will:

and the land shall be subdued before you ; which must be done before their return:

then you shall give them the land of Gilead for a possession ; which, no doubt, included that of Jazer too, since Jaazer, which is the same, is after mentioned as one of the cities built by the children of Gad, (Numbers 32:35) .