First you have to import ,
using Google.GData.Client;
using Google.GData.Documents;
into your solution. Then use the below code for download the document.

 DocumentsService service = new DocumentsService("testAttempt");   service.setUserCredentials("testDrive@gmail.com Email", "password");// Use the proper credential of your //gdrive

// Initialize a query object
 Google.GData.Documents.DocumentsListQuery query = new   Google.GData.Documents.DocumentsListQuery();

            query.Title = test.txt;// search the title begin with d
            query.TitleExact = true;


            DocumentsFeed feed = service.Query(query);// Query send to Google server

            foreach (DocumentEntry entry in feed.Entries)
            {
                if (entry.Title.Text == "test.txt") // you may check the file name
                {
                    // create the download uri
                    Uri documentUri = new Uri(entry.Content.Src.Content);
                    string DocsPath = documentUri.AbsoluteUri;
                        string resourId = entry.ResourceId.ToString();// if you need the unique resource Id..                  
                    HttpWebRequest webRequest = (HttpWebRequest)
                    HttpWebRequest.Create(documentUri);
                    GDataGAuthRequestFactory factory = (GDataGAuthRequestFactory)
                    service.RequestFactory;

                    webRequest.Headers.Add("Authorization", "GoogleLoginauth=" + factory.GAuthToken);

                    HttpWebResponse webResponse = (HttpWebResponse)
                    webRequest.GetResponse();
                     Stream stream = null;
                    int bytesToRead = 1024;
                    byte[] buffer = new Byte[bytesToRead];
                    byte[] downloadedData = new byte[0];
                    HttpWebResponse fileResp = (HttpWebResponse)webRequest.GetResponse();
                    if (webRequest.ContentLength > 0)
                        fileResp.ContentLength = webRequest.ContentLength;

                    stream = fileResp.GetResponseStream();

                    MemoryStream mstream = new MemoryStream();
                    int length;
                    while (true)
                    {

                        length = stream.Read(buffer, 0, bytesToRead);

                        if (length == 0)
                        {
                            break;
                        }


                        else


                            mstream.Write(buffer, 0, length);
                    }



                    downloadedData = mstream.ToArray();
                    HttpContext.Current.Response.ClearContent();
                    HttpContext.Current.Response.AddHeader("Content-Disposition", "attachment;filename=\"" + test.txt + "\"");

                    HttpContext.Current.Response.ContentType = "text/plain";// specify the type
                    HttpContext.Current.Response.BinaryWrite(downloadedData);
                    HttpContext.Current.ApplicationInstance.CompleteRequest();
                    HttpContext.Current.Response.Flush();
                    HttpContext.Current.Response.End();

                    stream.Close();
                    mstream.Close();

           }

}

thanks.