$ dnf install gdm
$ systemctl disable lightdm
$ systemctl enable gdm
$ reboot
https://unix.stackexchange.com/questions/86221/how-can-i-lock-my-screen-in-gnome-3-without-gdm
Fedora 29 cannot lock screen on desktop
$ dnf install gdm
$ systemctl disable lightdm
$ systemctl enable gdm
$ reboot
https://unix.stackexchange.com/questions/86221/how-can-i-lock-my-screen-in-gnome-3-without-gdm