How can I disable IPv6 as a Xfi Advanced gateway subscriber with gigabit unlimited

Either get your own router and put the gateway in bridge mode, or get your own modem and use the router (and save the rental fee.)

@Vecdid Thank you for reaching out to the Digital Care Team. You can use your own modem in bridge mode. You will not be able to retain xFi during this process. In early 2011, the Internet Assigned Numbers Authority (IANA) exhausted the global IPv4 free pool. Given this growth, it is important that adequate IP address resources are available to ensure that everyone can connect to the Internet. IPv4 will no longer be able to satisfy these needs.

Broader question on this same issue:  I need to disable IPV6 on both the WAN and LAN.  Does this require replacing the existing Technacolor Router/Modem/Wifi device with my own equipment?  And if I do this will the Flex still work?

I can't comment on the Technicolor router since I don't have one and have never used one, but generally speaking if you want to disable IPv6 on Comcast service you'll need your own router unless whatever problems IPv6 causes you are specific to some particular device/service and you can just disable IPv6 on the affected devices.  Like if it's breaking some Windows app you need for work you can probably just disable it on the Windows machine and not worry about the Comcast stuff.  You can put Comcast devices or owned cable routers in bridge mode and use them as a modem, but you'll still need a router that can turn IPv6 off.  You'll probably also have to be picky about cable wifi routers.  I don't recall an option to disable IPv6 on the old Arris cable WiFi router I used before I switched to separate components and business grade gear except for the cable modem.