下記ページ参照
　http://melan260.net/?p=530
下記URLに解決策
　http://forum.unity3d.com/threads/cant-use-gzipstream-from-c-behaviours.33973/
　ICSharpCode.SharpZipLib.dllが必要
using System;
using System.IO;
using System.Text;
using ICSharpCode.SharpZipLib.BZip2;
 
[System.Serializable]
public struct Zipper
{
    public static string ZipString(string sBuffer)
    {
        MemoryStream m_msBZip2 = null;
        BZip2OutputStream m_osBZip2 = null;
        string result;
        try
        {
            m_msBZip2 = new MemoryStream();
            Int32 size = sBuffer.Length;
            // Prepend the compressed data with the length of the uncompressed data (firs 4 bytes)
            //
            using (BinaryWriter writer = new BinaryWriter(m_msBZip2, System.Text.Encoding.ASCII))
            {
                writer.Write( size );
               
                m_osBZip2 = new BZip2OutputStream(m_msBZip2);
                m_osBZip2.Write(Encoding.ASCII.GetBytes(sBuffer), 0, sBuffer.Length);
               
                m_osBZip2.Close();
                result = Convert.ToBase64String(m_msBZip2.ToArray());
                m_msBZip2.Close();
               
                writer.Close();
            }
        }
        finally
        {
            if (m_osBZip2 != null)
            {
                m_osBZip2.Dispose();
            }
            if (m_msBZip2 != null)
            {
                m_msBZip2.Dispose();
            }
        }
        return result;
    }
 
    public static string UnzipString(string compbytes)
    {
        string result;
        MemoryStream m_msBZip2 = null;
        BZip2InputStream m_isBZip2 = null;
        try
        {
            m_msBZip2 = new MemoryStream(Convert.FromBase64String(compbytes));
            // read final uncompressed string size stored in first 4 bytes
            //
            using (BinaryReader reader = new BinaryReader(m_msBZip2, System.Text.Encoding.ASCII))
            {
                Int32 size = reader.ReadInt32();
               
                m_isBZip2 = new BZip2InputStream(m_msBZip2);
                byte[] bytesUncompressed = new byte[size];
                m_isBZip2.Read(bytesUncompressed, 0, bytesUncompressed.Length);
                m_isBZip2.Close();
                m_msBZip2.Close();
               
                result = Encoding.ASCII.GetString(bytesUncompressed);
               
                reader.Close();
            }
        }
        finally
        {
            if (m_isBZip2 != null)
            {
                m_isBZip2.Dispose();
            }
            if (m_msBZip2 != null)
            {
                m_msBZip2.Dispose();
            }
        }
        return result;
    }
}