gksu Removed From Ubuntu, Here's What You Can Use Instead suggests to use gvfs. Example:
gedit admin:///etc/default/apport
Bug #1740618 “Remove gksu from Ubuntu” : Bugs : umit package : Ubuntu saying
Debian has removed gksu. I recommend that we remove gksu and libgksu for Ubuntu 18.04 LTS also.
It is recommended that developers use PolicyKit to only use elevated privileges for the specific actions where it is needed.
It is recommended that users use the gvfs admin backend available in Ubuntu 17.10 and 18.04 LTS. You can do this with the admin:// prefix. For instance, instead of running gksu gedit or gksu nautilus to edit /etc/default/grub, navigate to admin:///etc/default/ and open the grub file.
Ubuntu 17.10’s default session (a themed GNOME on Wayland) does not support gksu. Ubuntu 18.04 switched back to using X instead of Wayland as default but it is expected that Wayland will once again be the default in 18.10.
policykit? gvfs?
Citation from more authoritative sources may be required to learn what the plan for Debian is.
That is because RootSudo - Community Help Wiki contradicts it and says
gksu has been replaced by pkexec, but even pkexec is being deprecated by the mainline Ubuntu developers. They have taken the position that file manipulation and editing under root should be restricted to the command line.
Also citation required.