Yep. Firewall, routing, dhcp, dns, everything you’d expect from a gateway device. Plain Debian (or really any distro) can do it all. With a 1gbps bi-directional connection fully saturated it will run at about 10% cpu on my very crappy low power Celeron CPU.
Plus, there’s no web UI full of janky and insecure CGI scripts to exploit, and software updates are forever (well, until x64 is deprecated, so basically forever).