GCM is replaced by FCM. Also, sure, some user might run pure Android and not have a centralized push notification system, but that is not the problem people us

GCM is replaced by FCM. Also, sure, some user might run pure Android and not have a centralized push notification system, but that is not the problem people usually might be experiencing. The power management and priority system is rather complicated on Android and incoming notifications might be deferred or not shown at all(to avoid wasting power on low priority notifications) depending on app, app usage and settings.