Thanks to Jimi for his comment to use the CopyTo() method instead! New function:

Public Function Decompress(ByVal bSource As Byte()) As MemoryStream ' Byte()
    Dim inStream As New MemoryStream(bSource)
    Dim gzip As New GZipStream(inStream, CompressionMode.Decompress)
    Dim outStream As New MemoryStream()
    gzip.CopyTo(outStream)
    Return outStream
End Function