DotNetZip isn't a wrapper on unmanaged zlib.
Depending on how the content was compressed, you may be able to decompress it with DotNetZip.
Check the documentation, In particular see the doc on Ionic.Zlib.ZlibStream, Ionic.Zlib.DeflateStream, and Ionic.Zlib.GZipStream .
there are code examples in the doc pages for these classes.
Good luck. The documentation is your friend.