Hello Sudhir,
In the GW level,
Once you redefine the standard service and select all the necessary model related things by creating a new project ( entities , associations & navigation etc ... ) , now you can do changes to the new model of the new project as per your needs and generate the run time artifacts.
Now go the newly create DPC_EXT class, redefine the necessary methods as per your needs and add your custom logic to the existing one, activate the class.
In /IWFND/MAINT_SERVICE add the newly created service by selecting the appropriate system alias name and save.
Click on GW Client button and test the newly created functionality.
For more info ->
https://help.sap.com/saphelp_fiori/fiori10_extend_en.pdf
Extensibility of SAP Fiori Apps
Regards,
Ashwin