Why does this Pasuk refer to Gershon and Merari as "Nos'ei ha'Mishkan" and Pasuk 21 to K'has, as "Nos'ei ha'Mikdash"?
Because, whereas Gershon and Merari carried the curtains and the planks, K'has carried the Aron and the other holy Keilim.
What was the procedure regarding the carrying?
Rashi and Ramban #1: As soon as Degel Yehudah began to travel, Aharon and his sons took down the Paroches and covered the Aron 1 (and the other holy vessels, 2 and arranged the poles) 3 and the b'nei Gershon and Merari dismantled the Mishkan loaded the wagons and followed Degel Yehudah. 4 Whereas K'has waited for Degel Reuven to travel, after which they picked up the holy Keilim and followed them. 5
Ramban #2 (citing a Beraisa): Gershon and Merari traveled behind Machaneh Reuven and K'has, behind Efrayim. 6
Rashi (on Pasuk 21): The Shulchan, the Menorah and the two Mizb'chos.
As described at the end of Naso.
As stated in this Pasuk.
As stated in Pasuk 21.
See Ramban, who reconciles the Beraisa with his first explanation, which fits better into the Pesukim.