I too am on pre-order and had similar questions. As best as I can tell, the confusion comes from the word "gateway," as though it is a fancy piece of equipment different from a modem or a router. AT&T calls its Arris BGWxxx box a gateway, but it is just a router. Every fiber installation needs to change over from fiber (light) to electrical, usually Ethernet cable. The box that does that is the ONT. AT&T installs one when you buy its fiber service, and Sonic installs its ONT when you but its native fiber service.
Routers take an electrical signal (Ethernet) in from the ONT (or from a DSL modem if the outside source is over copper), then spread it around the network by both Ethernet and wi-fi, per the connections you set up.
Unfortunately, AT&T put some of the necessary handshake in the Arris BGW box, so that as practical matter, you must connect it, not your own router, to the ONT. Then AT&T calls the box a gateway.
Sonic did the rational thing, putting this handshake in its ONT so that it can connect to nearly any router.