How can I add a hibernate option to the GDM 3 menu? I only have suspend there but I need hibernation as it does not eat the battery. (Just to make sure - I am talking about the menu in the greeter, not in the gnome-shell itself - the system is multi-user and every user is supposed to switch to greeter and hibernate when done). The hibernation itself is working just fine (from Gnome Shell menu or command line), just no option in GDM. Alternatively, custom keyboard shortcuts working under GDM would do the trick as well.
I found a partial solution - creating a hibernation keyboard shortcut working in GDM. First I ran the following commands to enable login for gdm user:
chsh -s /bin/bash gdm passwd gdm
followed by the password. The I logged in as gdm user and adjusted the keyboard shortcuts for gdm user. I would still like to get it to the menu as well but at least a partial workaround for the moment.
If switching to lightdm is an option, you can just enable hibernation normally (which you may have already done) like shown here. The lightdm top-right menu will contain the hibernate option, just like the in-Unity menu. You can log in to Gnome Shell just as well from lightdm.