Google Cloud Messaging is originally a platform to send notification for Android. They added an iOS SDK but using their SDK on iOS means that you are using both GCM and Apple Push notification service:
Push for iOS device => Google Cloud Messaging => Apple APNS => Device
If you are only developing an iOS app, it is best to use APNS directly. If you are already all you infrastructure set for GCM, then you may save you a bit of code by supporting only GCM to address both iOS and Android.
For more details, you may use this Official Google Document: https://developers.google.com/cloud-messaging/ios/client
Link below is a Google Cloud Messaging IOs Quickstart app, it demonstrate how to connect an IOS app to GCM and how to receive messages: https://github.com/googlesamples/google-services/tree/master/ios/gcm