See : http://aerogear-dev.1069024.n5.nabble.com/aerogear-dev-UPS-and-com-ganyo-gcm-server-td12015.html and http://aerogear-dev.1069024.n5.nabble.com/aerogear-dev-UPS-1-2-GCM-3-0-Topics-and-Google-s-rest-client-updates-td12027.html for discussion.
Basically we need to enhance the GCM rest-client library to support topics.