Adam Clarke on Numbers 32:34

The children of Gad built - Aroer - This was situated on the river Arnon, Deu 2:36; 2Kgs 10:33. It was formerly inhabited by the Emim, a warlike and perhaps gigantic people. They were expelled by the Moabites; the Moabites by the Amorites; and the Amorites by the Israelites. The Gadites then possessed it till the captivity of their tribe, with that of Reuben and the half of the tribe of Manasseh, by the Assyrians, 2Kgs 15:29, after which the Moabites appear to have repossessed it, as they seem to have occupied it in the days of Jeremiah, Jer 48:15-20.