Is it possible to register for and receive Voip pushes (PushKit pushes) on iOS using Google Cloud Messaging (GCM) service ?
No, GCM for iOS uses the standard APNS push messages on iOS, not the newer VoIP push messages of Apple PushKit. In this way it is similar to the majority of third party push notification services such as Azure Notification Hub.
To my knowledge, only PubNub supports sending push messages using a unified framework for both GCM and Apple PushKit (VoIP push).