Rashi on Numbers 4:22/רש"י על במדבר ד׳:כ״ב

1נשא את ראש בני גרשון גם הם TAKE ALSO THE SUM OF THE SONS OF GERSHON - "also" implies: even as I have commanded you regarding the sons of Kohath; i.e. to see how many of them have already reached the age to be included amongst those fit for service.