The sons of Gershon were the immediate male offspring of Gershon, who was the first son of Levi. They are listed according to their age, so Libni is the elder. By their families is ambiguous. It uses the same word for families as in verses 14 and 15, but the preposition by is confusing. It may be understood either as “with their clans” (New Jerusalem Bible), meaning “and their clans” (Jerusalem Bible), or “arranged by clans” (Translator’s Old Testament), meaning perhaps “family by family” (New English Bible). New American Bible has “as heads of clans.” Probably “and their clans” is the easiest way to interpret it. Good News Translation‘s “and they had many descendants” seems to be without warrant.

Quoted with permission from Osborn, Noel D. and Hatton, Howard A. A Handbook on Exodus. (UBS Helps for Translators). New York: UBS, 1999. For this and other handbooks for translators see here .