我在 ubuntu 18.04 上下载了 Upwork 桌面应用程序。当我用 Gdebi 包安装程序打开它时，我看到了这个错误：
我尝试gksu通过手动安装，sudo apt install gksu但出现此错误
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
如何在 ubuntu 18.04 上安装这个软件（我昨天下载了它，所以它是 upwork 网站上的最新版本）？