The requested method POST is not allowed for the URL

I am making an app that when button clicked, Sending a GCM message.

so In my server , I inserted my server code of nodejs.

and below is my server code(GCM_nodeDemo.js).

and when i post MY SERVER URL/GCM_nodeDemo.js at POST MAN , I got "The requested method POST is not allowed for the URL /GCM_nodeDemo.js"

How can I solve this Problem? (Below is GCM_nodeDemo.js CODE)

  var http = require('http');
  var express = require('express');

  var app = express();

  app.use(app.router);
  app.use(express.bodyParser());

  var gcm = require('node-gcm');

  var message = new gcm.Message();

  var message = new gcm.Message({
      collapseKey: 'demo',
      delayWhileIdle: true,
      timeToLive: 3,
      data: {
           key1: 'hi.',
           key2: 'hihi push demo'
            }
   });

  var server_access_key = '~';
  var sender = new gcm.Sender(server_access_key);
  var registrationIds = [];

  var registration_id = '~';
  registrationIds.push(registration_id);

  app.get('/GCM_nodeDemo.js',function(request,response){
      response.send("testa");
      console.log("testest");

  sender.send(message, registrationIds, 4, function (err, result) 
    {
    console.log(result);
    });
                                                      });

  app.post('/GCM_nodeDemo.js',function(request,response){
    response.send("testa");
    console.log("testest");
      sender.send(message, registrationIds, 4, function (err, result) 
      {
        console.log(result);
      });
                                                         });
  app.post('/push',function(request,response){
         response.send("testa");
         console.log("testest");
         sender.send(message, registrationIds, 4, function (err, result) 
          {
            console.log(result);
          });
                                              });
  /**
    * Params: message-literal, registrationIds-array, No. of retries, callback-function
  //  **/
  sender.send(message, registrationIds, 4, function (err, result) {
      console.log(result);
                                                                  });
  http.createServer(app).listen(52273,function(){
     console.log('Server Running');
                                                });