So, I install Ubuntu 17 clean on my laptop after the issues I had with drivers and immediately found out that gksu was not installed.

Installed that and tried to

gksudo nautilus

That failed and found out that Wayland had replaced the default of Xorg. Found an old Xauthority file from my backups and copied that back, which allowed me to get the popup window back for my gksu, but I couldn’t click it to enter the password :(

Then I found this article:

https://www.linuxuprising.com/2018/04/gksu-removed-from-ubuntu-heres.html

Which tells me I need to use the admin:/// file prefix instead to open something up as admin. Guess I’ll give it a go later.