recovering a broken GNOME-desktop in debian wheezy

Try doing this in terminal

sudo apt-get update
sudo apt-get upgrade #see if the package is held back
sudo apt-get purge gnome-session
sudo apt-get install gnome-session
startx

Source: http://ubuntuforums.org/showpost.php?p=10783266&postcount=5

first of all sudo apt-get purge gnome-session.

after all purging the gnome session make sure to clear packages by using bellow command sudo apt-get autoremove sudo apt-get autoclean

and than sudo apt-get install gnome-session it will work most the time when tasksel fail to install gnome.

i have tested this code on debian jessie and ubuntu lts 14.04.

i have this kind of crazy issue once i have messed up with gtk or multiple session at a time So also.

this will help you out form the session issue.

After upgrading from Squeeze to Wheezy a virtual machine running on KVM, I had to install "gnome-session-falback" in order to open successfully a session in gnome:

sudo apt-get install gnome-session-fallback