From RFC 1952, the GZip file header is structured as: +---+---+---+---+---+---+---+---+---+---+ |ID1|ID2|CM |FLG| MTIME |XFL|OS | (more-->) +---+---+---+---+---+---+---+---+---+---+ Looking at the output you've provided, we have: | Java | Go ID1 | 31 | 31 ID2 | 139 | 139 CM (compression method) | 8 | 8 FLG (flags) |...