$output = base64_encode(gzencode('Hallo Welt',9));

Public Shared Function UnZip(compressedText As String) As String
Dim gzBuffer As Byte() = Convert.FromBase64String(compressedText)
Using ms As New MemoryStream()
Dim msgLength As Integer = BitConverter.ToInt32(gzBuffer, 0)
ms.Write(gzBuffer, 4, gzBuffer.Length - 4)
Dim buffer As Byte() = New Byte(msgLength - 1) {}
ms.Position = 0
Using zipStream As New System.IO.Compression.GZipStream(ms, _
  System.IO.Compression.CompressionMode.Decompress)
zipStream.Read(buffer, 0, buffer.Length)
End Using
Return System.Text.Encoding.Unicode.GetString(buffer, 0, buffer.Length)
End Using
End Function