You have some stuff (task queues) built into GAE for messaging. Start with that.
The problem with running stand alone integration soltuions (Camel, Spring Integration, ActiveMQ..) on GAE is that they need to open tcp listeners and start threads, which is not really an option in GAE. At least not the standard java way.
You can run Camel to some point if you want to leverage the Camel DSL and such things. But all of Camel will not work. Camel does not include a messaging solution either.
The other option is likely to use some external messaging source, such as a RabbitMQ or ActiveMQ hosted somewhere else. Like EC2 or some cloud service.