Albert Barnes 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 fruits of the conquest made by Israel were assigned to Gad and Reuben by Moses, at their own request.

Because ... seated - Render, because there was the leader’s portion reserved, that is, there was reserved the fitting portion for Gad as a leader in war.

And he came ... - that is, he joined the other leaders to fulfill the commands of God regarding the conquest of Canaan (compare Numbers 32:17, 21, 32; Joshua 1:14). Moses regards the promise of the Gadites to do this as already redeemed.