We've removed the need for GCM since SMSSecure only deals with the SMS/MMS functionality.

I don't know - anyone got time to review the source? - but if they're only doing SMS, they might not need it; after all, SMS is already 'push', and it was only needed for push notifications for the data channel.

I think it is because they only handle SMS's. It doesn't have the push messaging part. You still need TextSecure for that.