Python decompressing gzip chunk-by-chunk
gzip and zlib use slightly different headers.
See How can I decompress a gzip stream with zlib?
Try d = zlib.decompressobj(16+zlib.MAX_WBITS).
And you might try changing your chunk size to a power of 2 (say CHUNKSIZE=1024) for possible performance reasons.
I've got a more detailed answer here: https://stackoverflow.com/a/22310760/1733117
d = zlib.decompressobj(zlib.MAX_WBITS|32)per documentation this automatically detects the header (zlib or gzip).