Rashi on Numbers 18:2/רש"י על במדבר י״ח:ב׳

1וגם את אחיך AND ALSO THY BRETHREN — i.e. the sons of Gershon and the sons of Merari.

2וילוו means THAT THEY MAY JOIN THEMSELVES UNTO THEE to warn also the strangers (the ordinary Israelites) not to come near to them.

3וישרתוך AND THEY SHALL MINISTER UNTO THEE by guarding the Temple gates and by appointing from among them (cf. Arakhin 11b) keepers of the Temple treasury and superintendents over these (Sifrei Bamidbar 116:1).