What kind of data is transmitted via Google Cloud Messaging (GCM)?
Threema uses Google Cloud Messaging (GCM) only to inform the app in the background about incoming messages. The app then fetches messages directly from the Threema servers. Neither contents nor details about messages are transmitted via GCM (the GCM payload is empty).
You can choose whether or not you would like to use Threema with Google Services. The option “Troubleshooting > activate Polling” allows you to use Threema without GCM. However, we don't recommend this.
The development of a custom push-service is difficult, since its reliability depends on numerous factors. We are therefore consciously using the recognized push services of Apple, Google and Microsoft. They are already installed on the vast majority of devices. Furthermore, channelling push notifications saves battery power.