Google does only allow 20 devices in one group, as explained here: https://developers.google.com/cloud-messaging/notifications

The maximum number of members allowed for a notification_key is 20.

This means there is no gain in using this for mapping our alias to GCM device groups. In fact much more code is needed to handle the differences, and at device #21 the traditional way of sending a push via tokens/registration_ids is needed.