Let's test out using Gnome…

eix -C -I gnome | grep "\[UI\]" | awk '{print $2}' | xargs emerge -pC

Or, with newer versions of eix:

eix -CI gnome --only-names | xargs emerge -C

Since eix supports category query easily, If no eix installed, then slowly:

emerge -ep world | grep "gnome" | gawk '{print $2.8}' | xargs emerge -pC

Well if u have qlist, this would remove all package include 'gnome':

qlist -I gnome -C | xargs emerge -pC

mourn for gnome, Hah!