A gateway proxy is not a proxy for some other class (JDK or otherwise), it is an implementation of a supplied interface created by the framework. Gateways are used by client classes to interact with the messaging system (send/receive messages) without the client code having any knowledge of the messaging system itself. A user class is never involved in a gateway; user classes can be used in other components such as service activators etc, which can (if necessary) be proxied by any mechanism, including CGLIB.
Perhaps if you can explain exactly what you are trying to do with these classes, someone will be able to help you.
Gary P. Russell
Spring Integration Team
SpringSource, a division of VMware