Mr Gu has kept a record of all Qin Suwens court proceedings and she died after meeting with him. Obviously he told her that Niazhi was her and his daughter. One more secret he is yet to reveal to his daughter.
Maybe her biological mother isn't Qin Yaoguang? But Suwen? the reason why He Zhichu and Nianzhi are very knowledgeable in law moreso in legal battles. . .