Rashi and Rashbam: Because Hashem was instructing Moshe to count the B'nei Gershon, to see how many had reached the age of Avodah, just as he had instructed him to count the B'nei K'has. ¹

Da'as Zekeinim, Hadar Zekeinim, Moshav Zekeinim #1 and Rosh: Because Gershon was the oldest son of Levi, so in the count from one month old, B'nei Gershon were counted first. However, in the count of those who were eligible to serve, the B'nei K'has, who carried the holy vessels, were counted first. And the B'nei Gershon thought that they would not be counted at all. ² Therefore the Torah stresses that this is not the case and that they too, would be counted.

Moshav Zekeinim #2: To dismiss the notion that, since the Levi'im are disqualified from the Avodah above the age of fifty only for carrying on the shoulders, but not for singing or working with [transporting the Mishkan via loading and unloading] wagons, and Gershon did not carry on the shoulder that they are counted even above the age of fifty.

Moshav Zekeinim #3: To stress that also they are important, just like the B'nei K'has.

Refer to Bamidbar, 4:2:2:1*.

Moshav Zekeinim: The Torah writes "le'Mishpechosam" first by K'has and Merari because, whereas Gershon comprised only two families ? Livni and Shim'i, both K'has and Merari comprised many families.