If you need something permissively licensed, I believe microG also has implementation of the notification protocol: https://github.com/microg/GmsCore/blob/cb9be

If you need something permissively licensed, I believe microG also has implementation of the notification protocol: https://github.com/microg/GmsCore/blob/cb9be8f682d7649dae23bcc1619f8c5717de5b9d/play-services-core/src/main/java/org/microg/gms/gcm/McsService.java#L556 https://github.com/microg/GmsCore/blob/cb9be8f682d7649dae23b... It's Apache 2.