Ideally you want to disable the "router" part of their "gateway" (disable routing, NAT, and DHCP, and the wifi AP) and use it like a modem and connect the "WAN" port of your router to it. Sometimes this is called "bridge mode"
I've found this article describing how to use it:
https://www.xfinity.com/support/articles/wireless-gateway-enable-disable-bridge-mode
The other option is to use a switch, but all you need is a basic Ethernet switch. I would not waste the ERX on that duty, but instead pick up a dedicated Ethernet switch - you can get one with 8 ports for like $20 on Amazon.
Either of these would work fine:
Actually, i think using Cat8 in this circumstance was the smart thing to do. It's not like there's a HUGE difference in price for cables. And if these are going in the wall (i.e. not easily replaced) I would go with the best you can. Who knows what speeds we might have in 10, 15, 20 years etc
So if you have a modem/router combo from comcast and all of the Cat8 cable is plugged into that, you can just plug one of the cat8 cables into your gaming pc and you are done.
If there are more cat8 cables/outlets then you have ports on the back of the router, then what you can do is get a switch. Get at least 1gb switch with enough ports on it for however many cat8 outlets you had installed.
If you literally just have the cat8 cable coming out of the wall and no outlet you'll want to get something like this: https://www.amazon.com/BOPLAT-Shielded-Ethernet-Plate-Punch/dp/B0B36DZ5MH
And then get another cable (cat6 is more then fine) to plug into the outlet and into your pc.
Switch: https://www.amazon.com/NETGEAR-8-Port-Gigabit-Ethernet-Unmanaged/dp/B07PFYM5MZ
Plug that into the router, and then plug all the cat8 you have running through the house into that.
And if you get this: https://www.amazon.com/NETGEAR-Nighthawk-8-Stream-Tri-band-Router/dp/B07P3FGKYD, put it centrally located in the house relative to the comcast router and plug one of the cat8 cables into it, it should work. You just need to put it into bridge mode - or if you don't put it in bridge mode, you need to disable the wifi on the comcast router. Or go with one of the mesh options already mentioned.