ATT Fiber is mostly GPON with some areas that are XGS-PON. They just started rolling out 2gbit and 5gbit service in one city.
There are a couple of bypass methods on the GPON network that let you use your equipment with the ONT and bypass the RG. I don't think I would bother, though, and I'd just use the 'IP Passthrough' DMZ to assign the public IP to your router. The only problem was the size of the internal connection table on the RG was historically very small and didn't clear quickly which really only causes problems if you torrent. The newer RGs don't have this problem.
On the XGS-PON network the ONT is integrated into the RG so it's not possible right now, but again the DMZ 'IP Passthrough' mode actually works well.
They support IPv6 and they give out a /56 if that matters to you.
