First, you should not be using
sudo crontab -e for this. That will edit the crontab file for the root user. You need this to run in your user crontab so that the gnome session that gets saved is yours. Drop the sudo and just run
crontab -e. If necessary add yourself to the
cron.allow file so that you are allowed to have a crontab file as your user. (Also remember to edit root's crontab again and remove that entry.)
Secondly you can't run things like gnome utilities from cron without hooking them up with the correct
$DISPLAY variables. The environment that cron passes on to it's children is not nearly as complete as a login shell and since it doesn't run inside your graphical login environment, it isn't wired up to it. Consider this: it's possible to have more than one graphical environment running. How would a system script know which one you want to operate on? It doesn't. You need to figure out the what DISPLAY it's running on and pass that to your command.
Lastly, this seems like ALL the wrong place to be doing this anyway. Why on earth would you want to periodically kill a session? Anything the user is doing at that momment is going to be nuked. Also cron runs whether or not the user is logged in, so it may not even always have anything to operate on.