how to zip files in windows phone 8.1?

Question

I am using the code following:

StorageFile file = await folder.GetFileAsync("recording.mp4");
            StorageFile file1 = await folder.GetFileAsync("capturing.mp4");
            StorageFile zipFile = await folder.CreateFileAsync("Record.gz", CreationCollisionOption.ReplaceExisting);
            //FileInfo
            using (var writeStream = await zipFile.OpenAsync(FileAccessMode.ReadWrite))
            {
                //FileRandomAccessStream fileStream = writeStream as FileRandomAccessStream;
                using (GZipStream Compress = new GZipStream(writeStream.AsStreamForWrite(), CompressionMode.Compress))
                {
                    using (IRandomAccessStream readStream = await file.OpenReadAsync())
                    {
                        Stream stream = readStream.GetInputStreamAt(0).AsStreamForRead();

                        stream.CopyTo(Compress);
                        await Compress.FlushAsync();
                    }
                    using (IRandomAccessStream readStream = await file1.OpenReadAsync())
                    {
                        Stream stream = readStream.GetInputStreamAt(0).AsStreamForRead();

                        stream.CopyTo(Compress);
                    }
                }
            }
I am using GZipStream to compress 2 mp4 file into a gz/zip file. After running the code, i create the gz file successfully, 
but i cant open it in my phone. So, i put the file in my PC, and i can open it while there is only a file without a extension name. 
There should be 2 mp4 in the gz file.
but after i add ".mp4" to the file it can be played on the PC, but the second mp4 is lost while the file size equals to the total
size of 2 mp4 files.
So, can someone tell me that how to zip files correctly in windows phone or how to solve my problems?
Thanks!~