I think Gödel undecidable sentence is always relative to a formal system (the title of the paper spoke about systems in the expressive power rank of Russell's Principia, of which he gives one particular example assuming it shows how his methods apply to the whole family of systems), but now Hilbert problem #6 still stands for the lack of a comprehensive axiomatization of Physics, as its modern heir the mass-gap millennium problem, that still also lingers, so we don't have a controllable notion of naked/absolute undecidability for physical phenomena or for arbitrary assertions unbound by explicit logic rules in general.