I have long used and like gksu/do, however it's long been unmaintained also and found to have some security bugs as well. However Debian has removed it from the software repositories, so it's no longer available in the normal sense and a supposedly better and newer model implemented to open graphical apps with root/sudo privileges, namely polkit ... = pkexec and ///admin this and that. Myself however, want things back as they were. I type "gksudo thunar" in a run dialogue-etc, gnome-keyring pops up, asks for my password and then launches the file-manager with priv's as I wanted.
So what I did ... I just added the Stretch/oldstable repo's back in sources.list,
Then "apt update" to refresh packages and installed gksu from there with "apt install -t oldstable gksu". Afterwards commented out the oldstable repo and refreshed packages again to be anal. That's a wrap, I have my gksu and gksudo cmds working once again as I prefer them.# Added to get gksu/do + depends from Stretch repos.
deb http://deb.debian.org/debian oldstable main contrib non-free
As mentioned this is unapproved and generally looked down upon. There are in fact better alternatives to this dorkishness, one of which is lxqt-sudo(available in stock repositories), does the same thing as gksu/do but as far as I know is actively maintain and doesn't have the known sec issue which gksu does. So in preference you(and likely I will) could install it instead and then add a bash alias for the thing to .bash_aliases or whichever file you maintain your aliases in. Because lxqt-sudo is aggravating to type. If polkit gives me any grief about it's use. I will disable/mask the thing with the "systemctl" command so that it shuts up.
PS, Also note, that if you use Gnome/Wayland, such may not work regardless (even with Xwayland blahblah. Poss even with polkit disabled/masked.) Dunno ... I however don't give a whit about Gnome or Wayland and am using Openbox and Xorg, shrugs. If were going to dork with Gnome it'd surely be Cinnamon or MATE desktop, more shrugs. Just a heads up fellows.