Push messages on GCM are going through immediately. The push server is not queuing up messages to send them all at once. I admit that could be helpful but Google is not doing this either.

The alarm manager - where apps can schedule to be run at some point in the future - is in fact grouping those events together. But that happens entirely independent of GCM. The Alarm Manager is part of Android where as the push service is a proprietary Google service.