I am pretty sure that I am also experiencing this problem with my Pace 5268AC Gateway. Although I am not connected to Gigabit (Sonic Fusion), I did recently upgrade my service from 50 MB to 75MB speed. Of course, this uses the AT&T lines and consists two bonded pairs coming into my Gateway from the street.
I also run DMZ mode to light up my Gigabit wired LAN and two WiFi nodes. My WiFi Router is setup to handle all my internal home wired network using DHCP, as well as being one of two WiFi access points. Also have been doing this for over ten years starting with AT&T Uverse service before switching to Sonic about three years ago. Although all the network details are relatively obscure to me, I stumbled through a number of forums in the past to figure out how to run my home network behind the supplied Gateway.
Now that I am connected to 75MB service, I only get close to that speed when doing the test directly from the Pace 5268AC. When performing a test from behind my WiFi Router, connected via my wired Gigabit LAN, I only get speeds less than 50MB.
When I called Sonic support to ask about settings or parameters on the Gateway that might be checked, they informed me that they do not support anything other than the standard configuration. Basically, Sonic does not support me setting up a network, wired or wireless, behind the Gateway. I think this is unreasonable. I know that few people have a more complex network behind their Gateway, but why make it so difficult to make it work?
My wish is the Gateway could be set up just like a DSL modem in bridged mode, so that all my network and related service could be handled by my own, easy to use, and supported WiFi and Router hardware. It can't be that difficult to design a basic modem to connect to the WAN via the fiber, or other wired connections.