When, in 754, Eoban left to accompany Boniface on their last missionary trip, Gregory was tasked with administering the diocese of Utrecht, which he did faithfully for the next twenty-three years until his death in 776.

(Because of the similarity of names and also because of a forged will, Addula has been frequently confused with Saint Adela of Pfalzel, daughter of Dagobert II of Austrasia, thus wrongly imputing to Gregory membership of the royal house of the Merovingians).

[2] In 738 Saint Boniface made his third journey to Rome; Gregory went with him and brought back many valuable additions for his library.

[3] The school of his abbey, the Martinsstift, a kind of missionary seminary, was now a centre of learning for many nations:[5] Franks, Frisians, Saxons, even Bavarians and Swabians.

When the murderers of Gregory's two half-brothers were sent to him by the civil magistrates to be put to what death he should think fit, according to the custom of the country at that age, which left the punishment of the assassins to the direction of the relations of the deceased person; he gave every one of them a suit of clothes with an alms, and dismissed them with good advice.

[1] Gregory welcomed Lebuinus of Ripon and entrusted him with the mission of Overijssel on the borders of Westphalia, and gave him Marchelm (or Marcellinus), a disciple of Saint Willibrord, as a companion.

[7] Some three years before Gregory's death, paralysis attacked his left side and gradually spread over his entire body.

A large portion of his head is in the church of Saint Amelberga at Susteren, where an official recognition took place on 25 September 1885 under the supervision of the Bishop of Roermond.