The Method:

public static void DecompressFile( FileInfo FileToDecompress, String
DestinationDirectory, Boolean DeleteOriginalFile)
{
    FileStream fsSource = null;
    FileStream fsDestination = null;
    GZipStream compressedStream = null;

    try
    {
        Byte[] buffer;

        fsSource = new FileStream( FileToDecompress.FullName, FileMode.Open,
FileAccess.Read, FileShare.Read);

        // The original file size may be obtained from the footer of the
compressed file
        buffer = new Byte[ 4];
        fsSource.Position = Convert.ToInt32( fsSource.Length) - 4;
        fsSource.Read( buffer, 0, 4);
        Int32 _OriginalFileSize = BitConverter.ToInt32( buffer, 0);

        // Read the decompressed file contents
        buffer = new Byte[ _OriginalFileSize];
        fsSource.Position = 0;

        compressedStream = new GZipStream( fsSource, CompressionMode.Decompress,
true);
        compressedStream.Read( buffer, 0, _OriginalFileSize);
        compressedStream.Flush();
        compressedStream.Close();

        // Write the decompressed data to a new file
        String _OriginalFileName = FileToDecompress.Name.Substring( 0,
FileToDecompress.Name.Length - 5);
        fsDestination = new FileStream( DestinationDirectory +
Path.DirectorySeparatorChar + _OriginalFileName, FileMode.Create,
FileAccess.Write, FileShare.Write);
        fsDestination.Write( buffer, 0, _OriginalFileSize);
        fsDestination.Close();
        fsSource.Close();

        System.Diagnostics.Debug.WriteLine( String.Format( "The file {0} was
decompressed as {1}.", FileToDecompress.Name, _OriginalFileName));

        if( DeleteOriginalFile)
        {
            FileToDecompress.Delete();
            System.Diagnostics.Debug.WriteLine( String.Format( "The following file
was deleted: {0}", FileToDecompress.Name));
        }
    }
    catch( System.Exception ex)
    {
        System.Diagnostics.Debug.WriteLine( String.Format( "The following
exception occurred within the application:\n{0}", ex.Message));
    }
    finally
    {
        if( fsSource != null) fsSource.Close();
        if( compressedStream != null)
        {
            compressedStream.Flush();
            compressedStream.Close();
        }

        if( fsDestination != null) fsDestination.Close();
    }
}