As per help.sap, you should include your enhancement in below BADI :
GBAPP_APV_PR_RDP
You can use this BAdI to change the runtime data that is returned by the Gateway service and displayed on the user interface of the Approve Requisitions
app. This includes the use of pictures, changing the list of attachments and notes displayed, and changing the workflow. For more information, see the BAdI documentation and the example implementation.
Regards,
Tejas