John Gill Commentary Numbers 3:21

"Of Gershon was the family of the Libnites, and the family of the Shimeites: these are the families of the Gershonites." — Numbers 3:21 (ASV)

Of Gershon [was] the family of the Libnites, and the family
of the Shimites
The first son of Levi: these [are] the family of the Gershonites ;
that were now numbered.