John Gill Commentary Numbers 32:4

"the land which Jehovah smote before the congregation of Israel, is a land for cattle; and thy servants have cattle." — Numbers 32:4 (ASV)

Even the county which the Lord smote before the congregation of
Israel
In which the above cities were, and perhaps some others not named: this was now in the hands of the people of Israel, being subdued by them, the conquest of which is ascribed unto the Lord, for the victory was of him; it was he that smote their enemies; and delivered their country into their hands; and now Moses, Eleazar, and the princes of the congregation, being the representatives of the people, had a right to dispose of it, and, which these two tribes request might be given to them, because, say they:

it is a land for cattle, and your servants have cattle ;
to stock it, with and great numbers of them; see (Numbers 32:1)