I think it would be reasonable to not depend on gdk-pixbuf[introspection] for something that is concerned with GTK-3. Perhaps there should not be any dependencies for seed. If I'm not mistaken that should be techincally possible, so the user can decide which components (and introspection thereon) he really wants. Reproducible: Always
As far as I can tell, x11-libs/gdk-pixbuf is not a dependency of dev-libs/seed.
Sorry, I did not look thoroughly enough. It seems that it's GTK-3 which depends on it.