John Gill Commentary Numbers 4:27

"At the commandment of Aaron and his sons shall be all the service of the sons of the Gershonites, in all their burden, and in all their service; and ye shall appoint unto them in charge all their burden." — Numbers 4:27 (ASV)

At the appointment of Aaron and his sons
As they shall order and direct:

shall be all the service of the sons of the Gershonites ;
everyone of them was to take their share, as Aaron and his sons should appoint them:

in all their burdens, and in all their service ;
what each of them should carry, and in what they should perform duty:

and you shall appoint to them in charge all their burdens ;
charge them carefully to keep all that were committed to them to bear, that none be lost, not a pin nor a cord.