As you can see in
http://bugs.gentoo.org/show_bug.cgi?id=304777 (it's a tracker bug, then, please don't comment on it, for any problem, open a separate bug report), progress is taking place for trying to get new gtk+-2.18 stuff stabilized soon *before* gnome 2.28.
In that bug report, you can see what bugs needs to be fixed before (see "Bug 304777 depends on:"). The idea is to stabilize gtk+-2.18/glib-2.22.4 before gnome 2.28 and, then, Gnome 2.26 (current stable) needs to work fine with new gtk+. Seems that it works fine for me since months but, I can't test every single app depending on gtk+. Because of this, would be nice if you could unmask the following stuff in your STABLE system:
x11-libs/gtk+-2.18.6
dev-libs/glib-2.22.4
dev-cpp/gtkmm-2.18.2
dev-cpp/pangomm-2.26.0
dev-cpp/glibmm-2.22.1
x11-misc/shared-mime-info-0.70
app-office/grisbi-0.6.0_rc2
app-i18n/scim-1.4.9-r1
x11-libs/wxGTK-2.6.4.0-r6
x11-libs/wxGTK-2.8.10.1-r5
and see if gtk+ apps work and compile ok against new version (if you have a fast enough machine, you can even run an "emerge -e world" ;-)).
Please file a bug on every problem you find related with this, and add me (pacho at gentoo dot org) to CC list for fast tracking them.
Thanks a lot for your contribution :-)