Charles Spurgeon Commentary Numbers 4:21-24

"And Jehovah spake unto Moses, saying, Take the sum of the sons of Gershon also, by their fathers` houses, by their families; from thirty years old and upward until fifty years old shalt thou number them; all that enter in to wait upon the service, to do the work in the tent of meeting. This is the service of the families of the Gershonites, in serving and in bearing burdens:" — Numbers 4:21-24 (ASV)

They were to bear the external coverings of the holy place. The most holy place was in the custody of the Kohathites; but the Gershonites were to carry as follows,