/usr/share/lightdm/lightdm.conf.d/60-lightdm-gtk-greeter.conf is in lightdm-gtk-greeter 1.8.4-0ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | [SeatDefaults]
greeter-session=lightdm-gtk-greeter
|