Got it!
Let me elaborate it...
/IWBEP/IF_MGW_APPL_SRV_RUNTIME~GET_ENTITYSET is one of method from framework, you can find this from "Runtime Artifacts" section of SEGW.
In side above method, we can find IT_ORDER table passing to OUTPUT_GET_ENTITYSET method. We can see our RFC/BAPI is calling this method. This is where you can customize further i.e. you can send IT_ORDER table to RFC/BAPI if possible or you place your code in this method.
Thanks,
Naveen Inuganti