All of the themes in gnome-themes-extras except for Gorilla use the smooth theme engine. Gorilla uses the industrial engine which isn't in portage, but once it is in portage, gnome-themes-extras should depend on it too.
Turns out this isn't a bug since gnome-themes-extras includes the smooth and industrial engines. But it is confusing to have two ways to install the same engines.
Yes, that's true. There was a time when the Gtk themes issue was messy and, from the user point of view, the administration of themes and engines was problematic. Fortunately for us, packages like gnome-themes and gnome-themes-extras make the gtk themes/engines handling a lot easier now. Chahces are gtk-engines-* will be deprecated some time in the future.