<p>Are System.IO.Compression.GZipStream or System.IO.Compression.Deflate compatible with zlib compression?</p>

They just compressing the data using zlib or deflate algorithms , but does not provide the output for some specific file format. This means that if you store the stream as-is to the hard drive most probably you will not be able to open it using some application (gzip or winrar) because file headers (magic number, etc ) are not included in stream an you should write them yourself.