Not GP, but I did something similar for a few years with an old T60. I used the expresscard slot onboard to connect a quad-port server NIC using a pcie riser.

Not GP, but I did something similar for a few years with an old T60. I used the expresscard slot onboard to connect a quad-port server NIC using a pcie riser. For OS, I tried a couple different options including PFsense, VyOS, centos with Shorewall, and eventually settled on good old Debian with iptables rules. What was nice about using an old laptop was that you get a built-in UPS that's good for at least an hour (with the screen off) so you almost never have downtime because of power outages (provided your modem still has power, and the node/dslam is still up).