[strcat]

Push notifications without GCM can be done without losing reliability or ruining battery life, but it's difficult. There are a few apps like Conversations doing it well but it's quite rare. GCM is still technically more efficient since it's reused across multiple apps, but it doesn't really matter especially without a large number of apps doing this.

There's already a built-in warning on modern Android because apps need to request a battery optimization exception just as they would one of the dangerous permissions. Otherwise, GCM is the only way they be woken during Doze / App Standby for push notifications. Apps can't simply poll anymore. It's unfortunate that even an app doing a great job like Conversations gets a warning / prompt about this but there's little that can be done about that beyond the OS whitelisting the keys for known good apps.

Conversations is pretty much the only viable option without GCM. It uses a take on the Signal protocol via XMPP (when supported by the other contact) so you helped to make that happen anyway.