Comment 17 for bug 237662

Even I had the same problem. I didn't know that the cause for gksu's misbehaviour was name change.
After trial and error, I discovered the solution. Here it is.
1. Go to System > Administration > Network.
2. Unlock the dialog box.
3. Goto Hosts tab.
4. Delete the entry with the IP address 127.0.1.1
5. Make a new entry with the IP address 127.0.1.1 and enter the new computer name in the "Aliases" area.
That's it.
Hope I was of some help.
Kshiteesh