Comment # 3 on bug 1197973 from Lars Martin Hambro

gnome-screensaver-command -l ; gdmflexiserver
If 'gnome-screensaver-command' is not a typo you can use command-not-found to
lookup the package that contains it, like this:
    cnf gnome-screensaver-command

(gdmflexiserver:15202): Gdm-WARNING **: 22:02:47.902: Unable to create
transient display: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The
name org.gnome.DisplayManager was not provided by any .service files
GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name
org.gnome.DisplayManager was not provided by any .service
fileslinux-xm8u:~/Desktop #