As I understand it, GCM uses long polling push to implement immediate push.
This costs battery dearly, compared with SMS or a technology similar to Blackberry push (presumaby using a specific GSM MAP) to inform the phone to contact the server for the payload over TCP.
What are the reasons for Google not appearing to pursue a GSM push mechanism to save battery life?
I don't know exactly, but may be because, some devices may not contain the SIM card, like some Tablets does not have SIM card facility, so sending SMS, will not be possible on those devices.
So to have a generalized solution for all devices, they might be supporting this mechanism.