Comment # 4 on bug 931314 from Dominique Leuenberger

I started debugging this a but this morning; findings so far:

SDDM successfully starts gnome-session (as defined in
/usr/share/xsession/gnome.desktop

the GNOME Session is starting up the auto start features, BUT
gnome-shell and gnome-settings-daemon (about the two most important components)
are skipped

from a jouranl debug:

GsmManager: Phase WINDOW_MANAGER
GsmManager: app /org/gnome/SessionManager/app1 is not for the current desktop
GsmManager: /org/gnome/SessionManager/app1 app-id:gnome-shell.desktop
is-disabled=1

This message appears due to OnlyShowIn=GNOME in gnome-shell.desktop.

glib (gio) compares XDG_CURRENT_DESKTOP against the OnlyShowIn and concludes in
this case that this does not match

Checking the environment of gnome-session, XDG_CURRENT_DESKTOP is defined as:
XDG_CURRENT_DESKTOP=
(it is defined, but empty - the same can be observed in a IceWM session)