Read Numbers 4:41 Using Other Translations
These are they that were numbered of the families of the sons of Gershon, of all that might do service in the tabernacle of the congregation, whom Moses and Aaron did number according to the commandment of the LORD.
This was the list of the clans of the sons of Gershon, all who served in the tent of meeting, whom Moses and Aaron listed according to the commandment of the LORD.
So this was the total of all those from the Gershonite clans who were eligible to serve at the Tabernacle. Moses and Aaron listed them, just as the LORD had commanded.