How to set up a GRE tunnel with netplan?

Question

GRE

Previously, on Ubuntu, the gre-tunnel could be configured using ifupdown, for example

auto tun1
iface tun1 inet static
    address <tunnel IP>
    netmask <tunnel subnet mask>
    pre-up iptunnel add tun1 mode gre local <local IP> remote <remote IP> ttl 255
    up ifconfig tun1 multicast
    pointopoint <remote tunnel IP>
    post-down iptunnel del tun1

Netplan is currently running by default. In principle it is possible to install ifupdown and work as before, but I'm wondering how to do the same with netplan