Numbers 3:18

And these are the names of the sons of Gershon by their families; Libni, and Shimei.

The verse belongs to the first of the two numberings of Israel that give the book of Numbers its name, the numbering in the early part of the book, within the first four chapters that record the numbering of the people, the numbering of the tribe of Levi, and the office of the various families of Levi.
It names Libni and Shimei as the sons of Gershon by their families.
Those families of Gershon formed one of the three divisions of the tribe of Levi assigned to service around the tabernacle.

The verse stands among the instructions assigning duties to the Levitical families, in which the Kohathites, not Gershon's sons, were appointed to carry the ark and sacred vessels on their shoulders.
The listing is part of the order for priests in their divisions and Levites in their courses for the service of God at Jerusalem written in the book of Moses, which the returned Jews followed when they appointed the temple service at its dedication.
The chapter shows that the Aaronic priesthood was established by law for its age, with the priesthood vested in the tribe of Levi until Christ's priesthood began.