COMPRESS uses a zlib compatible format - always with deflate algorithm, 32k window size, max compression and no dictionary - followed by checksum (4 bytes) and uncompressed data length (4 bytes).
For REBOL 2.x, this module uses a precalculated crc table, and for REBOL 3.x, it uses the available 'crc32 checksum method - r3 compress/gzip is bugged (wrong checksum), so it isn't used.