gksu Removed From Ubuntu, Here's The Recommended Replacement

The gksu package, which provides GTK+ front-ends for su (gksu) and sudo (gksudo), was removed from the Ubuntu 18.04 repositories (and Linux Mint 19 Tara), so it won't be available starting with this Ubuntu release.

gksu is used to allow elevating your permissions when running graphical applications, for example in case you want to run a graphical text editor as root to edit a system file, or to be able to remove or add a file to a system folder.

How to edit system files using a GUI without gksu?

Enter admin://. The recommended way of performing operations that require elevated permissions using a GUI is by using the gvfs admin backend, which is available in both Ubuntu 18.04 LTS (and Linux Mint 19 Tara) and 17.10, by using the admin:// prefix.

All you have to do is open Nautilus (or Nemo), press Ctrl + L so you can type in its address bar, and type admin:// followed by the path of the folder containing the file you want to edit, and enter your password. After doing this, you can use any GUI application to alter system files, like Nautilus to add / delete files or folders, Gedit (or Xed in Linux Mint 19) to edit text file, and so on.

Here's an example. Let's say you want to edit the Grub file. Instead of using gksu gedit /etc/default/grub, you can open Nautilus / Nemo and type: admin:///etc/default. You'll be asked for your password and after you enter it, you can open the grub file with Gedit (or Xed in Linux Mint 19), then save it.

Note that there are 3 forward slashes after admin: because 2 are required by default and the 3rd is from the path that follows, which begins with a slash: /etc/default. One more thing: in Nautilus, don't enter the direct path to a file but to a folder containing the file you want to edit.

In the same way you can run Gedit from a terminal to open a system file and edit it, for example: