/usr/share/xsessions/gnome-fallback.desktop is in gnome-session-flashback 1:3.8.0-1ubuntu11.
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 9 | [Desktop Entry]
Name=GNOME Flashback (Metacity)
Comment=This session logs you into GNOME with the traditional panel
Exec=/usr/lib/gnome-panel/gnome-session-flashback
TryExec=/usr/lib/gnome-panel/gnome-session-flashback
Icon=
Type=Application
X-LightDM-DesktopName=Unity
X-Ubuntu-Gettext-Domain=gnome-panel-3.0
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