The source of the subplot involving Gloucester, Edgar, and Edmund is a tale in Philip Sidney's Countess of Pembroke's Arcadia (1580-90), with a blind Paphlagonian king and his two sons, Leonatus and Plexitrus.
Besides the subplot involving the Earl of Gloucester and his sons, the principal innovation Shakespeare made to this story was the death of Cordelia and Lear at the end; in the account by Geoffrey of Monmouth, Cordelia restores Lear to the throne, and succeeds him as ruler after his death.