umbra ~ # echo "app-editors/gvim -gnome gtk" >> /etc/portage/package.use 
umbra ~ # emerge -uD world
Calculating world dependencies      ... done!                               

!!! Error: the >=gnome-base/libgnomeui-2.18 package conflicts with another package;
!!!        the two packages cannot be installed on the same system together.
!!!        Please use 'emerge --pretend' to determine blockers.

For more information about Blocked Packages, please refer to the following
section of the Gentoo Linux x86 Handbook (architecture is irrelevant):

http://www.gentoo.org/doc/en/handbook/handbook-x86.xml?full=1#blocked