Numbers 4:19-23

Rather, do this for them so that they will live and not die when they approach the most holy objects: Aaron and his sons shall go in and assign each of them to his work and to his load;

but they shall not come in to see the holy objects even for a moment, or they will die.” Duties of the Gershonites

Then the Lord spoke to Moses, saying,

“Take a census of the sons of Gershon also, by their fathers’ households, by their families;

from thirty years old and upward to fifty years old you shall count them: all who can enter to perform service, to do the work in the tent of meeting.