For sending notifications to android phone first you need to register yourself in google developer console

https://console.developers.google.com

After doing registeration in that you need to generate an API key from it

Code for GCM

		    string GoogleAppID = gsm.NotificationGoogleAppId;

                    //Project ID created in Google project
                    var SENDER_ID = gsm.NotificationSenderId;

                    //Registration Id created by Android App i.e. DeviceId
                    string regID = deviceId.Trim();

                    WebRequest webRequest;
                    webRequest = WebRequest.Create("https://android.googleapis.com/gcm/send");
                    webRequest.Method = "post";
                    webRequest.ContentType = " application/x-www-form-urlencoded;charset=UTF-8";
                    webRequest.Headers.Add(string.Format("Authorization: key={0}", GoogleAppID));
                    webRequest.Headers.Add(string.Format("Sender: id={0}", SENDER_ID));

                    string postData = string.Format("collapse_key=score_update&time_to_live=108&delay_while_idle=1&data.message={0} &data.time={1} &data.badge={3} &data.type={4} &data.id={5} &data.sound={6}&registration_id={2}", message, DateTime.UtcNow, regID, badgeCount, type, id, "default");

                    Byte[] byteArray = Encoding.ASCII.GetBytes(postData);

                    webRequest.ContentLength = byteArray.Length;

                    Stream dataStream = webRequest.GetRequestStream();
                    dataStream.Write(byteArray, 0, byteArray.Length);
                    dataStream.Close();

                    WebResponse webResponse = webRequest.GetResponse();

                    dataStream = webResponse.GetResponseStream();

                    using (StreamReader streamReader = new StreamReader(dataStream))
                    {
                        String sResponseFromServer = streamReader.ReadToEnd();
                        streamReader.Close();
                        dataStream.Close();
                        webResponse.Close();
                    }

This is the code where you have to pass the API key the Device UUID and the the Sender Id generated from the Google Console