gdk-pixbuf-2.42.2.ebuild has: emesonargs+=( $(meson_use gtk-doc gtk_docs) but meson_options.txt uses gtk_doc not gtk_docs: option('gtk_doc', description: 'Whether to generate the API reference (requires GTK-Doc)', type: 'boolean', value: false)
*** Bug 779142 has been marked as a duplicate of this bug. ***
Indeed, you're right. I didn't fix this (though I could have) in 2.42.2 because the fix would just uncover another problem in 2.42.4 which uses gi-docgen and has a missing python dependency.
*** This bug has been marked as a duplicate of bug 779142 ***