The carrier does not have to implement anything specific for Google. Apple's implementation shows how to do hybrid messaging apps without putting any burden on the carrier. They use the standard SMS system provided by the carrier where necessary so no changes needed by the carrier as they just process it as a normal SMS. And when both sides have Apple devices the messages are exchanged securely via IP which carriers can also process as normal IP packets. Google could easily do the latter securely while offering an opt-in (or opt-out like Apple) option to fall back to insecure SMS.