I was able to place the xscreensaver.desktop shown below in /usr/share/gdm/greeter/autostart to run xscreensaver at the login screen. I can see that the xscreensaver process is running as gdm user. When I try to activate xscreensaver I only get a blank screen and after moving the mouse I can see the gnome-shell clock. When I press enter I return to the login screen. Is there any troubleshooting methods I can use to help figure out why Xscreensaver is not working at the login screen?
Contents of xscreensaver.desktop:
[Desktop Entry] Type=Application Exec=/usr/bin/xscreensaver -nosplash Hidden=false Name=XScreenSaver