GCM(Google Cloud Messaging) registration token is a kind of id given by the GCM connection server to user/client that allow user to receive messages. To get registration token first we have to generate a unique number, it can be created while configure your API project, now given as Project number in Google Developer Console.

AsyncTask<Void, Void, String> task = new AsyncTask<Void, Void, String>() {

       @Override
       protected void onPreExecute() {
          super.onPreExecute();
                
       }

       @Override
       protected String doInBackground(Void... params) {

           try {
               InstanceID instanceID = InstanceID.getInstance(context);
               token = instanceID.getToken(context.getResources().getString(R.string.gcm_defaultSenderId),
                            GoogleCloudMessaging.INSTANCE_ID_SCOPE, null);

               } catch (Exception e) {

                    Log.d("TAG", "Failed to get token", e);
               }
                return token;
       }

       @Override
       protected void onPostExecute(String token) {

            Log.i("GCM Registration Token:", token);
       }
};
task.execute();