ResetableGzipOutputStream creates invalid gzip files when finish() and resetState() are used. The issue is that finish() flushes the compressor buffer and writes the gzip CRC32 + data length trailer. After that, resetState() does not repeat the gzip header, but simply starts writing more deflate-compressed data. The resultant files are not readable by the Linux "gunzip" tool. ResetableGzipOutputStream should write valid multi-member gzip files.
The gzip format is specified in RFC 1952.