server/usr/share/doc/netplan/examples/direct_connect_gateway_ipv6.yaml

network:
  version: 2
  renderer: networkd
  ethernets:
    eth0:
      addresses: [ "2001:cafe:face:beef::dead:dead/64" ]
      routes:
        - to: "::/0"
          via: "2001:cafe:face::1"
          on-link: true