Comment 4 for bug 316958

Yes, I just had a quick scan through the code and I can confirm that gnome-session only searches for desktop files in $XDG_DATA_DIRS/applications for launchers of components that are specified in "/desktop/gnome/session/required_components", so it wouldn't find gnome-wm.desktop - resulting in the error you see.

However, gnome-session still scans $XDG_DATA_DIRS/gnome/autostart and $XDG_CONFIG_DIRS/autostart when building the list of applications to start, so gnome-wm still gets autostarted. Therefore, I agree that gnome-wm.desktop should probably be installed to /usr/share/applications, but this is a low importance bug.