What is the admin:// prefix and how to make it work?

Looking for an alternative to gksudo in Debian, I stumbled upon [this article](https://itsfoss.com/gksu-replacement-ubuntu/) , which suggests to use the admin:// prefix in order to access files as root. E.g. instead of gksu gedit /etc/default/apport one is supposed to run gedit admin:///etc/default/apport I don't use gedit, but running leafpad admin:///etc/fstab on my Debian Buster simply opens an empty file. What am I missing? mtp:// is working on that machine, so I suppose GVFS support is present.