Kubuntu is a different distro and uses its own package management. The
default is KPackagekit. KDE does not use Software Centre, but you can
install it. Now in Ubuntu a DEB file when clicked on will open Software
Centre and no longer use Gdebi. If you do the same thing in Kubuntu then it
will open with Gdebi as happened in the past.

In many ways we are blessed in Kubuntu not to get everything that Canonical
develops.