GOG's hashdb is a very simple format providing checksums for each file from given product depot
goggame-GameId.hashdb
hashdb is a zip file with one entry (called the same as archive)
File starts with 3, 4-byte intigers. First two are always 12 and 1
The third intiger is number of records
Each record is a 1024 byte file path relative to install directory.
And 32 bytes of MD5 checksum.