Apperently, for the GCM Sender, this can not be done out of the box, we have to subclass the Sender Class as suggested here : https://groups.google.com/forum/#!topic/android-gcm/d2fFLgI3pA0
For APNs, a method is supplied to the service builder to add a proxy : http://notnoop.github.io/java-apns/apidocs/com/notnoop/apns/ApnsServiceBuilder.html#withProxy(java.net.Proxy)
Apperently, for the GCM Sender, this can not be done out of the box, we have to subclass the Sender Class as suggested here : https://groups.google.com/forum/#!topic/android-gcm/d2fFLgI3pA0
For APNs, a method is supplied to the service builder to add a proxy : http://notnoop.github.io/java-apns/apidocs/com/notnoop/apns/ApnsServiceBuilder.html#withProxy(java.net.Proxy)