The G64 file extension is associated with emulators of Commodore 64 legendary 8-bit computer sold in 80's and early 90's. File contains disk image captured from C64 diskettes or hard drives. It can be opened, or mounted in the Vice C64 emulator etc.
The G64 file extension is related to a video file format used by various security cameras to store captured video data. The *.g64 video files can be played in the Omnicast software video player for Windows.