/usr/share/xsessions/gnome-xorg.desktop is in gnome-session 3.28.1-0ubuntu2.
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 7 8 | [Desktop Entry]
Name=GNOME on Xorg
Comment=This session logs you into GNOME
Exec=gnome-session
TryExec=gnome-session
Type=Application
DesktopNames=GNOME
X-Ubuntu-Gettext-Domain=gnome-session-3.0
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