I downloaded the Upwork desktop App on ubuntu 18.04. When I open it with Gdebi package installer I saw this error:

I tried to install gksu manually via sudo apt install gksu but I got this error

Building dependency tree 
Reading state information... Done
Package gksu is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'gksu' has no installation candidate

How can I install this software on ubuntu 18.04 (I download it yesturday so it is the most recent version on upwork website) ?

Introduction

As Jeremy Bicha wrote:

Posted on April 18, 2018 by Jeremy Bicha — 8 Comments

Today, gksu was removed from Ubuntu 18.04, four weeks after it was removed from Debian.

So theoretically speaking there are no packages on Bionic Beaver 18.04 LTS, depending on gksu. But this is not true:

$ apt-cache rdepends gksu
gksu
Reverse Depends:
 gexec
 wicd-gtk
 spacefm-gtk3
 spacefm
 |menu

$ aptitude why gksu 
i menu Suggests gksu | kde-runtime | ktsuss

As you see some software, which are distibuted by third-party may depend on gksu package. So you can proceed installing gksu with method below on your own risk.

How to fix?

Possible solution is to install version from previous (17.10, artful) release.

Warning: do not execute the commands below if unsure!

* Short method (as @MK suggested)

wget http://mirrors.kernel.org/ubuntu/pool/universe/libg/libgksu/libgksu2-0_2.0.13~pre1-9ubuntu2_amd64.deb
sudo apt install ./libgksu2-0_2.0.13~pre1-9ubuntu2_amd64.deb

wget http://mirrors.kernel.org/ubuntu/pool/universe/g/gksu/gksu_2.0.2-9ubuntu1_amd64.deb
sudo apt install ./gksu_2.0.2-9ubuntu1_amd64.deb

* Long method

1. 
   

   Add artful repositories to the system:

   
   
   

   cat <<EOF | sudo tee /etc/apt/sources.list.d/artful.list
   deb http://archive.ubuntu.com/ubuntu/ artful universe
   EOF
   

   
   


2. 
   

   Update package cache

   
   
   

   sudo apt-get update
   

   
   


3. 
   

   Install gksu package

   
   
   

   sudo apt-get install gksu
   

   
   


4. 
   

   Remove artful repository from system for safety

   
   
   

   sudo rm /etc/apt/sources.list.d/artful.list
   sudo apt-get update
   

   
   

Test

Test gksu (should work on Xorg-sessions)

gksu-properties # check that it has "Authentication mode" to "sudo"
gksu date
gksudo date

Notes

After installation the following packages will be marked as obsolete (locally-installed): gksu, libgksu2-0. But they will work as expected.

Mixed repositories

Adding the artful repositories to a bionic system is an absolutely horrible idea. I cannot stress enough how much suffering the notion, alone, of running with mixed repositories will bring. There shall be misery and gnashing of teeth to the end of the age and beyond.

Masochists, please at least use protection (Apt Pinning).

Better ideas

• Wait for new releases that do not depend on outdated packages. (Patience is a virtue that will prevent embarrassing visits to the Help Center with conf-files, or worse, stuck where they don't belong.)




• Rebuild packages without unnecessary dependencies.




• Find PPAs with the packages you want.




• Put a wrapper -script- around pkexec.




• Faux play -with faux packages- and equivs.

Notes

• kdesudo has been dropped as well.




• lxqt-sudo is still around, but didn't work for me.




• Something called gosa? No idea what it is.




• Might as well play sudoku until this mess is figured out.

Gksu has been removed from the Ubuntu 18.04 LTS repositories. Though, there are a few ways to install it besides. One way is to use N0rbert's answer.

Install Gksu manually

Gksu only has one required dependency not included in 18.04 repositories. To install this way, just download the two .deb files. Here are the links:

64 bit packages:

Gksu

libgksu2-0

Ubuntu 18.04 LTS removed support for 32 bit packages, so I won't be including those.

If you are using Gksu for your own purposes (not for a dependancy), try this substitute program

Pkexec is a good substitute for Gksu. It is included with the system. Like I said, go ahead and try it as a replacement for gksu or gksudo.

Notes

Pkexec is a program that was installed on Ubuntu 18.04 LTS on both of my machines. I had not installed it before hand. Though I did notice that it was not on the apt repositories either, it is preinstalled, so I feel it is a good substitute candidate.

There are other programs similar to Pkexec and Gksu, like Kdesudo. I assume Kdesudo is included with the KDE desktop environment.

Edit:

Kdesudo has been removed with Gksudo.