John Gill Commentary Numbers 3:23

"The families of the Gershonites shall encamp behind the tabernacle westward." — Numbers 3:23 (ASV)

The families of the Gershonites
The two before mentioned, the Libnites and Shimites: shall pitch behind the tabernacle westward ;
this was their situation when encamped; they were placed in the rear of the camp of the Levites, between the tabernacle and the camp of Ephraim, which was westward also, (Numbers 1:18) .