盖 has three distinct meanings: (1) to cover (盖被子, cover with a blanket), (2) a lid (瓶盖, bottle cap), and (3) to build (盖房子, build a house). The 'build' meaning is colloquial — 建 (jiàn) is more formal.
grammar
盖章 (gài zhāng) means to stamp a seal — very common in Chinese bureaucracy. Official documents need a red stamp (公章) to be valid.
