Retrieves the #GFileInfo constructed from the GZIP header data
of compressed data processed by @compressor, or %NULL if @decompressor's
#GZlibDecompressor:format property is not %G_ZLIB_COMPRESSOR_FORMAT_GZIP,
or the header data was not fully processed yet, or it not present in the
data stream at all.
Retrieves the #GFileInfo constructed from the GZIP header data of compressed data processed by @compressor, or %NULL if @decompressor's #GZlibDecompressor:format property is not %G_ZLIB_COMPRESSOR_FORMAT_GZIP, or the header data was not fully processed yet, or it not present in the data stream at all.