Deuteronomy 3:12 (ESV)

12 “When we took possession of this land at that time, I gave to the Reubenites and the Gadites the territory beginning at Aroer, which is on the edge of the Valley of the Arnon, and half the hill country of Gilead with its cities.

In the distribution of the land to the Transjordanian tribes (east of the Jordan), Reuben and Gad are given the southern half, made up of what was called the tableland in Deuteronomy 3:10 and half of the hill country of Gilead. This territory was roughly the territory that had previously been Sihon's (Deuteronomy 2:36).