John Gill Commentary Numbers 3:18

"And these are the names of the sons of Gershon by their families: Libni and Shimei." — Numbers 3:18 (ASV)

And these [are] the names of the sons of Gershon, by their families Or to Gershon belonged two families, called after the names of his sons, who were now numbered, namely, Libni and Shimei; and who are elsewhere mentioned as his sons, (Exodus 6:17); and from hence were the families of the Libnites and Shimites, as in (Numbers 3:21).