This is the service
`abodah  (ab-o-daw')
work of any kind -- act, bondage, + bondservant, effect, labour, ministering(-try), office, service(-ile, -itude), tillage, use, work, wrought.
of the families
mishpachah  (mish-paw-khaw')
a family, i.e. circle of relatives; figuratively, a class (of persons), a species (of animals) or sort (of things); by extens. a tribe or people -- family, kind(-red).
of the Gershonites
Gershunniy  (gay-resh-oon-nee')
a Gereshonite or descendant of Gereshon -- Gershonite, sons of Gershon.
to serve
`abad  (aw-bad')
to work (in any sense); by implication, to serve, till, (causatively) enslave, etc.
and for burdens
massa'  (mas-saw')
burden, carry away, prophecy, they set, song, tribute.