/usr/share/xgreeters/lightdm-gtk-greeter.desktop 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 3 4 5 6 | [Desktop Entry]
Name=LightDM GTK+ Greeter
Comment=This runs the GTK+ greeter, it should only be run from LightDM
Exec=lightdm-gtk-greeter
Type=Application
X-Ubuntu-Gettext-Domain=lightdm
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