As described in Introduction, you can modify two processing points for a gateway--Preprocess and Postprocess.
No handlers are provided by default for the Preprocess or Postprocess step, and, therefore, no handlers are listed by default in the Available List. If you have uploaded a handler, you can select it and move it to the Configured List.
To apply a user-written handler for these configuration points, you must first upload the handler, as described in Uploading user-defined handlers. (Select Gateway instead of Target for step 2). Then perform the following steps:
You can further modify the Configured List as follows: