John Gill Commentary Deuteronomy 3:12

"And this land we took in possession at that time: from Aroer, which is by the valley of the Arnon, and half the hill-country of Gilead, and the cities thereof, gave I unto the Reubenites and to the Gadites:" — Deuteronomy 3:12 (ASV)

And this land, which we possessed at that time
Or took possession of, having conquered it; for it still remained in their possession:

from Aroer, which is by the river Arnon :
on the borders of Moab, from thence as far as Gilead was the land which was taken from Sihon king of Heshbon, (Deuteronomy 2:36)

and half Mount Gilead, and the cities thereof :
which were taken from Og king of Bashan, (Deuteronomy 3:10)

gave I unto the Reubenites, and to the Gadites ;
at their request, on certain conditions to be performed by them, afterwards repeated.