John Gill Commentary Numbers 4:22

"Take the sum of the sons of Gershon also, by their fathers` houses, by their families;" — Numbers 4:22 (ASV)

Take also the sum of the sons of Gershon
As well as of the sons of Kohath; for though Gershon was Levi's eldest son, yet his posterity were not excused from the service of the tabernacle, and bearing things of it in journeying:

throughout the houses of their fathers, by their families ;
all in their several houses, and in the several families in those houses, that were of the age next mentioned, were to be numbered.