Sforno on Deuteronomy 33:21/ספורנו על דברים ל״ג:כ״א

1וירא ראשית לו, he was also entitled to this, although when the tribe of Gad chose lands formerly belonging to Sichon or Og, a land which was not part of the “Holy Land,” there had been additional motivation to that of having good grazing land for their flocks, namely that their part of the land would contain Moses’ burial place. This is the meaning of the reference to מחוקק, “the legislator” in our verse.

2ויתא ראשי עם, this was where the leaders of the people came together at the head of the nation in order to conquer the land.

3צדקת ה' עשה ומשפטיו עם ישראל, this was where the legislator, מחוקק, performed the righteous deeds of Hashem, by explaining to them G’d’s Torah in detail. This was also where the people of Israel were judged, and where Moses argued with the people about the application of the various social laws of the Torah. The term is found in Samuel I 12,7 where the prophet Samuel tells the people to line up so that he can not only judge the people but explain the righteousness of the laws of G’d, i.e. צדקות ה'.