What shows up if you check the history on the terminal? If you are not familiar with the history in terminal just pull it up and type "history". Seeing how you were using the gksu command you may need to use "sudo history" to see the history related to root permissions.
Not sure that error messages are kept there but I would take a look at it.
What does the `/.xsession-errors file have to say? If you are not familiar with "hidden" files they start with a . and are found in your /home/<user name> directory and are made visible by hitting Ctrl + h. Same key strokes will hide them again.
Dell XPS 420 Core2 Quad Q 6600, audigy5.1, Radeon HD 6450 - currently 4 320Gb HDD, Debian Squeeze for secure use, Debian testing for daily use, Debian Sid for fun.