Numbers 4:28

28 This is the service of the Gershonite clans at the tent of meeting. Their duties are to be under the direction of Ithamar son of Aaron, the priest.

Read Numbers 4:28 Using Other Translations

This is the service of the families of the sons of Gershon in the tabernacle of the congregation: and their charge shall be under the hand of Ithamar the son of Aaron the priest.
This is the service of the clans of the sons of the Gershonites in the tent of meeting, and their guard duty is to be under the direction of Ithamar the son of Aaron the priest.
So these are the duties assigned to the Gershonite clans at the Tabernacle. They will be directly responsible to Ithamar son of Aaron the priest.