HI Peter,
the document mentioned by Simon actually tells the steps the redefine a Gateway Service.
Please refer to section 2.2 for the detailed steps. This tells you how to redefine a gateway service.
First you need to create a new Zproject in your workspace in SEGW. Then Right-click on the Data Model folder and choose Redefine Service OData Service (GW) . Here you need to enter the technical name of the service that you want to redefine. At the end of this process you will have a new service which you need to register. You can write your custom logic in class *****DPC_EXT or class ****MPC_EXT.
Section 2.2 of the mentioned document tells exactly the same process.
Regards,
Atanu
