Мусар к Бамидбар 4:22

נשא את ראש בני גרשון . This Parshah corresponds to the מחנה לויה, the encampment of the Levites. The counting of the Kehatites already was reported in Parshat Bamidbar, because the Priests were part of the Kehatites. Since they carried the Holy Ark on occasion, just as the Kehatites did most of the time, the latter were included in the מחנה שכינה, the encampment of the Priests. This Parshah however, deals exclusively with the camp of the Levites. It commences with the Gersonites since גרשון was the eldest of the sons of Levi and on his account all the Levites merited to take the place of the firstborn who had up to that time performed the priestly functions for the nation.

Another statement we find in the Midrash there is: "If someone whispers to you; was not Gershon the firstborn? Why did the Torah accord superior rank to Kehat and his descendants and counted the Gersonites only afterwards? – Answer such inquiry by pointing out that the Kehatites carried the Holy Ark which is the Holy of Holies, and because Aaron who performed service in the Holy of Holies was a descendant of Kehat." Gershon, by comparison, was holy, but not "holy of holies." In order to demonstrate that Gershon had not forfeited his status as the firstborn amongst his brothers, the Torah writes: "נשא את ראש בני גרשון". Just as the Torah employs the expression ראש, when it instructs the Kehatites to be counted, so it employs the same expression when the count of the Gersonites is to be taken. (This term is missing when the Merarites are being counted in 4,29). Seeing that the Torah uses the expression גם הם, "also them," when it introduces the count of the Gersonites, one might have interpreted this as proof that they are considered inferior to the Kehatites.