I can not get registration ID from Android GCM

Do you have GCMIntentService defined correctly at the root package of your app?

<receiver android:name="com.google.android.gcm.GCMBroadcastReceiver" 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="my_app_package" />
  </intent-filter>
</receiver>

Make sure that "my_app_package" is identical to the main package of your app, or your id will return an empty string.

Also notice that

    GCMRegistrar.register(this, "...");

is asynchronous. Therefore, calling GCMRegistrar.getRegistrationId(this) immediately after that is not reliable, so you should avoid it.

The id will arrive via broadcast callback to your GCMIntentService, as a part of the registration procedure. from there you can then store the gcm/fcm key anywhere you like, and use it in other parts of the application (usually on the server).

If your application launches first time, you have to wait for OnRegistered callback on your GCMIntentService.java file.

GCMRegistrar.register(this, SENDER_ID);
// Android does NOT wait for registration ends and executes immediately line below
// So your regId will be empty.
regId = GCMRegistrar.getRegistrationId(this);

GCMIntentService.java:

@Override
protected void onRegistered(Context c, String regId) {
    // You can get it here!
}

Edit: Newer version of GCM library (which bundled with google play services library) waits for response and returns the Registration ID. So this answer is for older GCM libraries.