The Riddles' dad's dad is the father of the Riddles' dad. He and the Riddles' dad's mom are seen behind a tree in a photo at the end of I Am Rukmini Devi.
It isn't clear which of them is a direct descendant of Great-Great Grandma Riddle, since Great-Great Grandma Riddle's son is not shown as an adult, and it isn't clear if he had a son or daughter.