Problem: You want to log in to a Gnome desktop on a remote Ubuntu 20.04 host.
Complication: You can only reach the host via SSH.
Further complication: A bug in Gnome currently makes it impossible to enable Gnome's built-in screen sharing if you connect via SSH. ☹
Solution: Use TigerVNC and an SSH tunnel, and log into that desktop! Here's how, minus the 2+ hours of head-scratching and swearing that accompanied my long trip down this short path today.