If you have a different GL device that does get a public IP then I’d try @admon 's suggestion. Otherwise you need a VPS for Headscale. See @eric 's suggestion. If you don’t want to bother with a VPS then look into signing up for Tailscale. There’s a free tier of some sort.
There’s addn’l configuration needed if you want you use your GL device as the access to the Public Internet via your Tailscale/Headscale connected devices (eg: phones, tablets, etc. – ‘exit node’):