Why does the Torah not tell us what they did with the gold, like it does regarding the silver and copper?
Ramban (on Pasuk 21): Because it was not known exactly how much gold went into each of the K'lei Kodesh, which were all under the jurisdiction of Elazar. 1
Riva (21), Hadar Zekenim, Da'as Zekenim: The only full Keilim made from the gold were the Menorah and its Keilim, they were a small amount 2 (one Kikar). Almost all was to plate the Kerashim and other Keilim.
Tosfos ha'Shalem (27:2, citing R. Yehudah ha'Chasid): It is impossible to weigh metal plating attached to wood.
Oznayim la'Torah: Moshe took account only of the silver and copper components of the Mishkan that were under the jurisdiction of the B'nei Gershon and Merari - who were governed by Itamar - but not of the golden Keilim that were transported by the B'nei K'has but were not under their jurisdiction, since they only carrried them after they had been prepared and covered by Aharon and his sons; 3 nor were they allowed in the Heichal once they had placed the Keilim when they encamped. 4
Refer also to 38:21:1:3*.
Riva: For the same reason, the Torah does not mention that some of the copper was used for clasps to connect the two sets of goats' hair curtains (26:11).
See Bamidbar, 4:16.
See Oznayim la'Torah who elaborates at length.
Why does the Torah list five kinds of gold Keilim - Choch, Nezem, Taba'as, Kumaz and Kol Klei Zahav?
Moshav Zekenim: These corresponded to the five kinds of afflictions that they suffered in Egypt - Perech, Merer, Inuy, Lachatz and Avodah Kasheh.
