right now gtk2 means "Use gtk+-2.0.0 over gtk+-1.2 in cases where a program supports both." and it was good some (long) time ago. at this moment gtk2 is practically standard in gtk-based aplications, while gtk1 is becomming more and more obsolete and more and more users don't want it in their systems. there's no simply way to have gentoo with gtk2, but without gtk1 based programs. programs with optional gui in gtk2 have "gtk" in IUSE, so user never know if it's gtk1 or gtk2 gui. whats worse - having "-gtk +gtk2" in USE can't guarantee that programs with optional gtk2 gui'll be builded with it. my suggestion is to change meaning of this flag to just "Adds support for x11-libs/gtk2" or something like that. flag gtk could stay as it is or change into "Use gtk+-1.2 over gtk+-2.0.0 in cases where a program supports both."
USE=gtk support gtk 1.x USE=gtk2 support gtk 2.x USE=gtk gtk support gtk 1.x and 2.x, use 2.x if both is present somethink like that would make sense and also imply that USE="-gtk gtk2" and USE="gtk -gtk2" works.
old request.. must be dupable somewhere
see bug #106560
