What does Gweddill mewn tangnefedd nana mean in English?

It's a flawed translation of 'rest in peace nana'. 'Gweddill' means rest but not in the sense of resting but rather in the sense of 'the remainder'. The correct translation would be 'gorffwys mewn tangnefedd nana' although you'd usually use 'hedd' rather than 'tangnefedd' as tangnefedd is usually used to mean serenity rather than peace.