ONT rather than Gateway

you gotta grin and live with it.  unlike docsis (cable) modems, you need to use the gateway AT&T supplies.  you can put your mesh wifi on it, or behind another router search "passthrough mode) in this forurm...

I have the 1000 Mbps fiber but do not use the gateway, I have a Mesh system that works great. I would like to get rid of the gateway but need an ONT to convert fiber to Ethernet. Does AT&T have one?

Yes AT&T offers 2 standalone ONTs: Nokia 010 and the Nokia 020. Since you have a BGW320 they will not want to install a standalone ONT even if you manage to get a Dispatch on Demand (DoD) truck roll and the tech has either the Nokia 010 or the Nokia 020.

Your option is to use Google or https://www.dslreports.com/forum/uverse to look into how to use a third-party SFT+ ONT. You currently only have an SFP+ Optical transceiver and the BGW320 has the rest of the ONT implemented in SoC in unit.

Note: there are 2 types of AT&T PON options for Fiber, GPON (Green) and XGS-PON (Red). You will need to check your current SFP+ Optical transceiver to see if it has a Green Plastic ring on the removal clip or a Red Plastic ring.

Dave