Comment 63 for bug 214370

This is not properly a bug but rather an unimplemented feature, as explained in the upstream gnome bug #528812. The code to execute the scripts at logout has been submitted to that bug by someone but nobody is finishing it up.

That's the status, no point adding entries in /etc/gdm/PostSession/Default, because nothing will read it, or hoping for a bugfix; someone has to implement the feature.