gdm-2.26 onwards, /etc/env.d/99xdg-gdm is installed setting XDG_DATA_DIRS to /usr/share/gdm . Without xdg-utils, the XDG_DATA_DIRS environment variable is not set properly [90xdg-data-base; XDG_DATA_DIRS=/usr/share], leading to g_get_system_data_dirs() not working.
Most stable users are not hit by this since if XDG_DATA_DIRS is completely unset, g_get_system_data_dirs() falls back to /usr/share (see glib-$PV/glib/gutils.c)
This results in a non-functional desktop; with gnome-session not being able to find metacity, nautilus, gnome-panel. Starting those manually from the console leads to warnings that they can't find various icons and themes.
I have confirmed that adding a custom env.d to define XDG_DATA_DIRS fixes the problem.
2.20 contains a dep on xdg-utils, but for some reason 2.26 and 2.28 don't. I've added the dep to fix this bug.