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.
shall pitch
chanah (khaw-naw')
to incline; by implication, to decline (of the slanting rays of evening); specifically, to pitch a tent; gen. to encamp (for abode or siege)
behind
'achar (akh-ar')
the hind part; generally used as an adverb or conjunction, after (in various senses)
the tabernacle
mishkan (mish-kawn')
a residence (including a shepherd's hut, the lair of animals, figuratively, the grave; also the Temple); specifically, the Tabernacle (properly, its wooden walls)
westward
yam (yawm)
from an unused root meaning to roar -- sea (-faring man, (-shore), south, west (-ern, side, -ward).