Helpful Information

• You can test Google Cloud Messaging using this test application*. Push notifications do not require account registration.
• You can add an account through the system settings. Some applications might ask you to do so, if you don't.
• Apps that use Cloud Messaging must be installed after GmsCore, or else they will not work. Some applications that can run with microG GmsCore is installed in the correct order: TextSecure/Signal, Play Music, YouTube
• If you are using AdAway, make sure to put mtalk.google.com on your whitelist, or else problems are likely to occur when using Google Cloud Messaging. Thanks @benstyle1 on XDA for the hint.

*app is not always accurate, consider testing with a different, possibly newer application