Enhancing Standard Gateway Services

Hi Experts,

We are deploying FIORI app in our landscape,

We have succesfully deployed My Travel Request and is working end to end.Now we are trying to enhance with a new
field on Get entityset.

Have anyone enhanced the standard Gateway services with a new field from SAP Backend to Gateway Modeling.

I have started with redefining the Method,but stuck with how to define a new field in Model Class.Please let me know,
anyone have done this.

Thanks for your support.