Something

Calculating dependencies... done!

Dependencies could not be completely resolved due to
the following required packages not being installed:

>=x11-drivers/nvidia-drivers-1.0.8178 required by media-video/nvidia-settings-1.0.20051122-r3

Have you forgotten to run `emerge --update --newuse --deep world` prior to
depclean?  It may also be necessary to manually uninstalled packages that no
longer exist in the portage tree since it may not be possible to satisfy their
dependencies.