Brother Gabriel is a Corbettite monk. He seems to be a resident of the Corbettite Depot Fortress of St. Szpac. He overhears Agatha, Violetta, Krosp, Zeetha, and Wooster discussing Agatha's plans to visit Paris and their speculation about whether those who ambushed, attacked, and are pursuing the Wyrm of Limerick are after Agatha. He decides they should see the Abbot, Father Gerät, and takes them to him.
He is always shown wearing, or holding, his characteristic pince-nez glasses.
He also seems to know something ✣ about breeds of cat.