Select the service and choose sap gateway client. In the gateway client select service implementation. You should see the package which you can then open in se80 and you will find the gw service builder projects, which you can open in segw. You should also note that the services are proxies so depending on the system setup the gw project may exist on a different system. This may also not work if they are not configured.
Regards,
Jamie
SAP - Technology RIG