Google Cloud Messaging - Help figure out how App Store app is generating GCM token for their private API

There is an app in the Apple App Store as well as the google play store. The app uses Google Cloud Messaging or (GCM). In order to make requests to their private API, the app first obtains a GCM token from google api. It then submits that GCM token to their private api along with the request. I already know how to make requests to their private api but I'm not quite sure how the GCM token is being obtained. I am unable to view the requests in 'Charles Proxy' or 'Fiddler' because the app developers have also implemented 'Certificate Pinning' All I need to know is HOW their app is requesting a GCM token. That is all. I don't need help with anything else. Please do not offer anything else. Please do not bid unless you know what I'm talking about as it is will no doubt be a waste of my time and yours.