x11-themes/gnome-themes includes "!x11-themes/gtk-themes" in its DEPEND section, but I'm guessing that this should really be something like "!>=x11-themes/gtk-themes-2". The x11-themes/gtk-themes package has two slots, one for GTK1 themes, and one for GTK2 themes. I'm guessing that it's only the GTK2 slot which would cause conflicts... Reproducible: Always Steps to Reproduce: 1. emerge <gtk-themes-2 2. try to emerge gnome-themes 3.
same here additional info: on emerge -Du world portage tries to update gtk-themes-1* to gtk-themes-2* even with adding gtk-themes-2* to /etc/portage/package.mask (various variants tried) and then runs into the Blocker from gnome-themes-2*
i removed gtk-themes from the tree, that should resolve this problem.