I have the same Residental Gateway. Using pfSense+ on my end. The BGW320-500 is fiber capable. I assume you're using fiber? If so you cannot hook it into ONT because the RG is the ONT. In my case I get raw fiber into a PON module that hooks into the RG. Best you can do in this case is set the RG to "passthrough mode" via web UI (192.168.1.254).
There is no double nat. Passthrough mode has worked as expected for me. The one issue I have is that the RG will maintain firewall states, so it limits you to the RG hardware for those states. I have a pretty large home network though, tons of devices, IoT, etc, and it has been stable.
Latency seems decent. I have an AT&T fiber 2gb symmetrical connection and a ping to google from my Netgate pfSense machine is around 10-15ms.