The seat here spoken of, probably referred to the situation of Gad, on the other side Jordan Deuteronomy 4:43. Joshua dismissed the Gadites with a blessing after the war was over; see Joshua. Joshua 4:12 with Joshua 22:7-8.
Related Commentaries of Deuteronomy 33:21
Deuteronomy 33:21
21 And he provided the first part for himself, because there, in a portion of the lawgiver, was he seated;d and he came with the heads of the people, he executed the justice of the LORD, and his judgments with Israel.