Information on SAP Gateway of Hub Architecture

Hello Guys,

I have one doubt and need your help on this.

I am going to develop an OData using the Gateway  HUB Architecture with backend development.

For example : I have created the OData service in my crm backend system (Lets suppose ICD) using tcode SEGW and i am going to register it to system located(Lets say PGD) at DMZ.

Now i am deploying  my ui5 application in PGD system.

So when i am using my UI5 Application its going to hit the PGD System , from there how the ICD system gets called , where my actual logic has been written and the data is fetched.

In other way my question is how the PGD system is going to call ICD System.

Please explain.

Thanks and Regards,

Vikash