Question: When did Richard De Clare, 6Th Earl Of Gloucester's mother die?
Answer: 17 January 1240

Isabel Marshal (9 October 1200 – 17 January 1240) was a medieval English countess. She was the wife of both Gilbert de Clare, 4th Earl of Hertford and 5th Earl of Gloucester and Richard, 1st Earl of Cornwall (son of King John of England). With the former, she was a great grandmother of King Robert the Bruce of Scotland