Deuteronomy 33:21 Commentary: Rashi, Sforno, Or HaChaim & Tur HaArokh

וירא ראשית לו AND HE SAW THE FIRST PART FOR HIMSELF — i.e. he saw good to take for himself territory in the land of Sihon and Og, the subjugation of which was the beginning (ראשית) of the conquest of the Land,

וירא 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.

וירא ראשית לו. "He chose the first portion for himself." Moses means that actually the ראשית, i.e. the Torah, should have been his share as Moses the lawgiver was buried in his territory. Alternatively, Gad became custodian of a חלק, a small part of the lawgiver, i.e. Moses' earthly remains securely hidden. The reason why Moses was buried there, i.e. on the East Bank of the Jordan was ויתא ראשי עם, in order for the generation of the Exodus, the heads of the nation, to eventually be resurrected together with Moses in order to be allowed to cross the Jordan. The reason that generation is referred to as ראשי עם is that they were the generation whom G'd acquired as His people.