custom spring integration outbound gateway implementation

How to specify service implementation for outbound gateway in Spring Integration? If outbound gateways are for bidirectional communication and generally used to provide integration with external systems and service activators are for local service calls then how to implement outbound gateway for custom transport/external system? Gateway namespace allows setting service interface for inbound gateway, but what about service implementation for outbound gateway?

<int:gateway id="cafeService"
         service-interface="org.cafeteria.Cafe"
         default-request-channel="requestChannel"
         default-reply-channel="replyChannel"/>