Aqui está o meu problema interessante. A notificação do Android que vem do GCM não está mostrando o título e o conteúdo (somente mostra o nome do aplicativo e, quando clicar, abra a MainActivity) quando o aplicativo não estiver em execução.

Mas quando o aplicativo está aberto, está mostrandocom sucesso título e conteúdo. Qual pode ser o problema? Estava funcionando sem problemas e não mudei nada.

Manifesto:

 <uses-permission android:name="android.permission.INTERNET" />
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
    <uses-permission android:name="android.permission.ACCESS_PHONE_STATE" />
    <uses-permission android:name="android.permission.ACCESS_WIFI_STATE" />
    <uses-permission android:name="android.permission.INTERNET" />
    <uses-permission android:name="android.permission.WAKE_LOCK" />
    <uses-permission android:name="android.permission.VIBRATE" />
    <uses-permission android:name="com.google.android.c2dm.permission.RECEIVE" />
    <uses-permission android:name="com.package.xxx.permission.C2D_MESSAGE" />
    <permission android:name="com.package.xxx.permission.C2D_MESSAGE" android:protectionLevel="signature" />

    <receiver
        android:name="com.google.android.gms.gcm.GcmReceiver"
        android:exported="true"
        android:permission="com.google.android.c2dm.permission.SEND" >
        <intent-filter>
            <action android:name="com.google.android.c2dm.intent.RECEIVE" />
            <action android:name="com.google.android.c2dm.intent.REGISTRATION" />
            <category android:name="com.package.xxx" />
        </intent-filter>
    </receiver>

    <service
        android:name=".Service.GcmService"
        android:exported="false" >
        <intent-filter>
            <action android:name="com.google.android.c2dm.intent.RECEIVE" />
        </intent-filter>
    </service>

GcmService.java:

import android.app.NotificationManager;
import android.app.PendingIntent;
import android.content.Context;
import android.content.Intent;
import android.os.Bundle;
import android.support.v4.app.NotificationCompat;
import android.text.TextUtils;
import android.util.Log;

import com.google.android.gms.gcm.GcmListenerService;
import com.package.xxx.Activity.ReadNormal;
import com.package.xxx.R;


public class GcmService extends GcmListenerService {

    public GcmService() {

    }

    @Override
    public void onMessageReceived(String from, Bundle data) {

        Log.d("GCMService", data.toString());

        String type = data.getString("type", "");

        if(type.equals("news")) {
           showNewsNotification(data);
        }

    }

    private void showNewsNotification(Bundle data) {

        String neId = data.getString("neId");

        if(TextUtils.isEmpty(neId)) {
            return;
        }

        int id = Integer.valueOf(neId);

        NotificationCompat.Builder mBuilder =   new NotificationCompat.Builder(this)
                .setContentTitle(data.getString("neTi"))
                .setContentText("Click to read more.")
                .setSmallIcon(R.drawable.ic_launcher)
                .setAutoCancel(true);

        Intent i = new Intent();
        i.putExtra("neSi", data.getString("neSi"));
        i.putExtra("neUr", data.getString("neUr"));
        i.putExtra("neTi", data.getString("neTi"));
        i.putExtra("neIm", data.getString("neIm"));
        i.putExtra("neId", id);
        i.setClass(this, ReadNormal.class);
        i.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);

        /***/
        PendingIntent pi = PendingIntent.getActivity(getApplicationContext(), 0, i, PendingIntent.FLAG_UPDATE_CURRENT);
        mBuilder.setContentIntent(pi);

        NotificationManager mNotificationManager = (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
        mNotificationManager.notify(0, mBuilder.build());

    }

    @Override
    public void onDeletedMessages() {

    }

    @Override
    public void onMessageSent(String msgId) {
    }

    @Override
    public void onSendError(String msgId, String error) {
    }

Obrigado.

Registra quando o aplicativo está sendo executado.

 D/GCMService: Bundle[{neId=4663755, neIm=http://icdn.posta.com.tr/editor/HD/30/1/2016/fft2mm7549077.jpg, neSi=Posta, neTi=Erdoğan: Rusya sonucuna katlanır, neUr=http://www.posta.com.tr/turkiye/HaberDetay/Erdogan--Rusya-sonucuna-katlanir.htm?ArticleID=324647, type=news, notification=Bundle[{e=1}], collapse_key=com.tekmobil.guncelhaber}]

Registra quando o aplicativo NÃO está sendo executado.

(empty, there is no log)