Here the author speaks of "descending" as if it were "coming." Alternate translation: "descend from Gershon" (See: [[rc://en/ta/man/translate/figs-metaphor]])
"Libnites" and "Shimeites" are the name of clans, named after the head of their family. The "Gershonites" is the name of people who descended from Gershon. (See: [[rc://en/ta/man/translate/translate-names]])