Thought I would share this since I've seen it asked about before without a clear answer and took me awhile to figure out.
On Gnome 3 to change the GDM screen time format issue the following command with either 12h or 24h to switch time formats.
And issue this command to change the GDM lock screen background. Use any .jpg image but make sure it is in a directory to which GDM will have access. /usr/share/backgrounds is a safe bet.
On Gnome 3 to change the GDM screen time format issue the following command with either 12h or 24h to switch time formats.
Code:
sudo -u gdm dbus-launch dconf write /org/gnome/desktop/interface/clock-format "\"12h\""Code:
sudo -u gdm dbus-launch dconf write /org/gnome/desktop/background/picture-uri "\"file:///usr/share/backgrounds/gnome/Stripes.jpg\""