During the short reign of his successor, abbot Heriward (987-990), the monks voluntarily relinquished their right of exemption in favour of Bishop Notger of Liège, who was well disposed towards the monastery.
Under Thietmar flourished the famous chronicler Sigebert of Gembloux (1030-1112), who in a neat Latin style wrote a chronicle of the world from 381-1111, a history of the abbots of Gembloux and other historical works of great value.