"This building is a bank."
Translation:Bangunan ini adalah bank.
Actually about the use of bangunan and gedung, there isn't an exact rule I guess. Just to be remembered, all gedung are bangunan, but not all bangunan are gedung. Like, you may say a house is a bangunan, but it's kinda weird to say that a house is a gedung. Meanwhile, a department store can be both bangunan and gedung. But for skyscrapers, "gedung pencakar langit" is an official translation, it's like a set already. So for some words, they're already a set and you need to memorize it as a whole, not separately like gedung, pencakar, and langit. For non-formal usage, it doesn't really matter if you say "bangunan" instead of "gedung" when referring to skyscrapers, people won't misunderstand you. But again, it is the official translation so you should use this in formal occasions like maybe a business meeting, or writing a journal.