So I've got a few ideas for how to implement this, but I hope some people more experienced with the platform can give some feedback before.

Quick Background:
In UPS right now we have a concept of categories.  A single UPS message can be broadcast to a bunch of devices which are subscribed to this category.  Google now supports this for GCM on Chrome, iOS, and Android so UPS can send a single message to GCM and GCM will broadcast that to up to a million devices.
End Quick Background

So first, how do we switch between sending a message to each device in a category to sending a topic message to GCM?  

In we are using the clientInstallationService to build a string of deviceTokens based on the variant and message criteria.  Is there any reason we can't create a "topicToken" which will be recognized later by GCMPushNotificationSender?  Another benefit to making this change here is that if we have over a million subscribers to the category we can just default to the default messaging.

There is also an open issue of whether or not we will update the clients to filter based on what category a message was sent to.  To do this we will have to include the category information in the message when we send it to devices going forward.  In GCM a topic message includes this information.  This means that if we have over a million subscriptions in the topic we will need to fall back to using the category information anyway.

Continuing on from the thread of falling back, it is possible for a topic message to fail to send because there are too many subscribers.  How would UPS handle regenerating the messages as deviceToken instead of topicToken messages?

Of course if someone has a better idea than "topicTokens" I'm all ears.