He did not exercise priesthood and it was Hugh (Hugo), who reportedly had the "character of wise and austere life" according to Albertus Magnus, who ruled de facto over the spiritual part of the Church of Nantes.
In 982, Guerech signed a treaty with Count William IV of Poitou, who confirmed the possessions of Nantes south of the Loire — the pagi of Herbauges, Tiffauges and Mauges — which his father, Alan II of Brittany, had obtained in 942.
[1] The following year, Guerech wento to the Court of the King of West Francia, Lothair to pay homage.
[3] The same year, Guerech's wife Aremburga had a strategic fortress built in Ancenis, the Château d'Ancenis.
Guerech's life, as well as his brother Hoël's, is mainly known through the Chronicle of Nantes, compiled in the 11th century and which is unfavourable to the Bretons in general and to the inhabitants of Rennes in particular.