It's just two broadcast receivers (one for receiving the push token, another for receiving actual notifications), and one broadcast sender to ask GSF to give you a token. This code is so trivial it's not even worth separating into a library.

Here's how you request a push token:

    Intent intent = new Intent("com.google.iid.TOKEN_REQUEST");
    intent.setPackage("com.google.android.gms");
    intent.putExtra("app", PendingIntent.getBroadcast(context, 0, new Intent(), PendingIntent.FLAG_IMMUTABLE));
    intent.putExtra("sender", FCM_SENDER_ID);
    intent.putExtra("subtype", FCM_SENDER_ID);
    intent.putExtra("scope", "*");
    intent.putExtra("kid", "|ID|1|");
    context.sendBroadcast(intent);

    <receiver android:name=".PushNotificationReceiver" android:exported="true" android:permission="com.google.android.c2dm.permission.SEND">
        <intent-filter>
            <action android:name="com.google.android.c2dm.intent.RECEIVE" />
        </intent-filter>
    </receiver>
    <receiver android:name=".api.PushSubscriptionManager$RegistrationReceiver" android:exported="true" android:permission="com.google.android.c2dm.permission.SEND">
        <intent-filter>
            <action android:name="com.google.android.c2dm.intent.REGISTRATION"/>
        </intent-filter>
    </receiver>

The second one is where you get push tokens. There will be a "registration_id" extra string which is your token. It may start with "|ID|1|" (the "kid" parameter from the request, not quite sure what it does), in which case you need to remove that part.

You want to refresh your push token every time your app gets updated and also just periodically if you haven't done it in a while. I do it every 30 days.

If you need something permissively licensed, I believe microG also has implementation of the notification protocol: https://github.com/microg/GmsCore/blob/cb9be8f682d7649dae23b...

It's Apache 2.