Charles Ellicott Commentary Deuteronomy 33:21

"And he provided the first part for himself, For there was the lawgiver`s portion reserved; And he came [with] the heads of the people; He executed the righteousness of Jehovah, And his ordinances with Israel." — Deuteronomy 33:21 (ASV)

The first part. —The first territory conquered by Moses was distributed between Reuben and Gad, and the half tribe of Manasseh.

A portion of the lawgiver is interpreted by Rashi as the field of the “burial-place” of the lawgiver. But this can hardly have been in the mind of Moses.

He came with the heads of the people. —The Gadites with their companion tribes passed over Jordan to the conquest of Canaan by Moses’ order.