Where does “རྒྱ་སེར” come from?

རྒྱ་སེར (Tibetan) comes from Tibetan རྒྱ, from Proto-Tibeto-Burman r-gya-n.

Ancestry of “རྒྱ་སེར”, step by step

Step	Language	Word	Meaning
1	Tibetan	རྒྱ	extent, size, area, width; vastness,...
2	Proto-Tibeto-Burman	r-gya-n	—