gtk+ 3.x is not used by gnome versions in portage (2.x). If you don't know if you need it, then you don't need it. You will be happy as long as portage doesn't want to remove gtk+ 2.x which is what most programs use. I don't even know if there's anything in portage at all that will require 3.x.
-- Jesús Guerrero Botella
