TgO Num3:25 :וֻמַטְרַת בְנֵי גֵרְשוֹן בְמַשכַן זִמנָא‏ : the watch of the Gershonites was in the "tent of meeting"
See the entire entry for maṭṭrā, maṭṭartā n.f. custody, care