Comment 9 for bug 234884

I suggest to add a "Run task manager" entry to gnome's panel context menu, just like in Windows. This would be much easier to discover for users than any keyboard shortcut.

The bug you linked is marked as a duplicate of a bug #11160 which was about Gnome misbehaving when ctrl-alt-del was assigned as a shortcut, not about just using that shortcut by default.