Why does the Torah write "Gam heim" in connection with Gershon?
Rashi: Hashem was instructing Moshe to count the Bnei Gershon, to see how many had reached the age of Avodah, just as he had instructed him to count the Bnei Kehas. 1
Da'as Zekeinim, Hadar Zekeinim, Moshav Zekeinim #1, Rosh: Gershon was the oldest son of Levi, so in the count from one month old, B'nei Gershon were first. In the count of those proper to serve, Bnei Kehas were counted first, for the primary Avodah (carrying the most Kadosh matters) is upon them. Do not think that Bnei Gershon will not be counted at all.
Moshav Zekeinim #2: One might have thought that Leviyim are disqualified from Avodah above the age of 50 only for carrying on the shoulder, but not for singing or working with [transporting the Mishkan via loading and unloading] wagons. Gershon did not carry on the shoulder. One might have thought that they are counted even above 50.
Moshav Zekeinim #3: Also they are important like Bnei Kehas.
Rashbam: In 4:2.
Why does it say "Naso" regarding Bnei Gershon and Kehas, but not regarding Merari?
Moshav Zekeinim: Gershon was the Bechor, and Kehas carried the Aron. Merari did not have either attribute.
Regarding Gershon, it says "l'Veis Avosam" before "l'Mishpechosam", and regarding the others, the order is reversed!
Moshav Zekeinim: Bnei Gershon were only two families