root@localhost linux # emerge -Du world --pretend

These are the packages that I would merge, in order:

Calculating world dependencies ...done!
[blocks B     ] <=x11-themes/gnome-themes-2.8.2 (is blocking x11-themes/gtk-engines-2.6.3)
[blocks B     ] <app-admin/gnome-system-tools-1.1.91 (is blocking app-admin/system-tools-backends-1.2.0)

Try removing the blockers...
# emerge --unmerge gnome-themes-2.8.2 gnome-system-tools-1.1.91

(I had to do # emerge --unmerge gnome-themes-2.8.2 gnome-system-tools)