Comment 3 for bug 773295

Exactly the same problem here, after using the alternate CD to install Ubuntu 11.04 with the command line only and installing the package lubuntu-desktop afterwards on a memory constrained system, gksu uses the su instead of sudo authentication method by default.

But I was able to solve the problem by running gksu-properties using the user account and not using the root account. So instead of running "gksu gksu-properties" try running "gksu-properties" and change the authentication method. Worked for me.

(IMHO, this has nothing to do with PolicyKit as applications like Synaptic currently lack integration with it. That is the reason they have to use gksu even though PolicyKit is preferred in the first place.)