John Gill Commentary Numbers 4:28

"This is the service of the families of the sons of the Gershonites in the tent of meeting: and their charge shall be under the hand of Ithamar the son of Aaron the priest." — Numbers 4:28 (ASV)

This [is] the service of the families of the sons of Gershon ,
&c.] Which they should execute when the camp and tabernacle were in motion:

in the tabernacle of the congregation in the court ;
where they should take up their burdens, or which lie in things respecting the tabernacle; this was the part they were to be employed in, namely, in taking care of the hangings of the tabernacle

and their charge [shall be] under the hand of Ithamar the son of Aaron
the priest ;
the things committed to their care were delivered by him to them, and they were under his inspection and direction.