using (DeflateStream gzip = new DeflateStream(fs, CompressionMode.Compress))
                {
                    byte[] buf = Encoding.UTF8.GetBytes(this.txbSource.Text);
                    gzip.Write(buf, 0, buf.Length);
                    gzip.Flush();
                }

解压只需要这样：

            gzip = new GZipStream(new MemoryStream(buf), CompressionMode.Decompress);
            using (StreamReader reader = new StreamReader(gzip))
            {
                this.txbTarget.Text = reader.ReadToEnd();
            }

如果从文件解压，只需要把MemoryStream换成一个FileStream就行了。
当然，需要加：using System.IO.Compression;