I haven't read through all of the replies, but you cannot eliminate the AT&T gateway. First off, the newer models like the one shown have the ONT built-in. Secondly, the fiber service requires certificate-based authentication, so even if you clone the AT&T gateway's MAC address on your pfSense it will not work.
The previous workarounds also do not work like they did with previous AT&T gateways and separate ONT module. If you want any fiber plan over 1 gig, then you will be issued the same gateway as in the original post. Also note the LAN ports are all 1 gig with the exception of first port which is blue and will support 5 gig, 2.5 gig, or 1 gig.