Зачем вам это нужно?
public static byte[] Decompress(this byte[] data)
{
var ms = new MemoryStream(data);
var s = new GZipStream(ms, CompressionMode.Decompress);
var output = new MemoryStream();
byte[] buffer = new byte[8192];
int read = 0;
while ((read = s.Read(buffer, 0, buffer.Length)) > 0)
{
output.Write(buffer, 0, read);
}
return output.ToArray();
}