Both the client and server are on protected segments of the gateway, correct?
Without knowing the exactly details provided and the specific security requirements, it's difficult to provide specific advice.
However, you have two basic options: by port or by URL.
To allow access to a "URL," in general, involves a Custom Application/Site object, which are inspected on the standard HTTP/HTTPS ports plus the proxy port (8080) by default (ports can be added to this).
If the URLs are not HTTP specifically (i.e. they are HTTPS), then for those URLs to be properly inspected, you also need to use HTTPS Inspection.
This requires deploying a trusted CA certificate on the relevant endpoints, which is a bit of a cumbersome process on iOS devices without some sort of Mobile Device Management solution.
If the application uses Certificate Pinning and/or requires mutual TLS authentication, HTTPS Inspection will not work.
If you cannot or don't wish to use HTTPS Inspection, then you open the relevant TCP/UDP ports for the application.