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).

If you have a different setup that is not fiber maybe you'll have more luck with a bypass, but I think you will need the RG regardless for auth: https://docs.netgate.com/pfsense/en/latest/recipes/authbridge.html

What firmware is your BGW320 currently on? There's a method for newish firmwares that should work on the 320, and are confirmed working for the BGW210.

No easily accessible guide for it yet, but for OPNSense and PFSense themselves, there's a simpler bypass available now. It still requires certificates. PFSense has an auth bridge mode that does not require certificates, but requires 3 interfaces and for your modem to still be plugged in.

You will need to connect the ONT ethernet directly to the WAN port for a bypass to work.