Gilles was born in 1420 to the incumbent duke, John V of Brittany and his wife, Joan of France, the daughter of the French king, Charles VI.
A letter from 5 July 1445 was intercepted by agents of the Duke, who refused to pardon his younger brother until the intervention of his uncle, the connétable de Richemont.
Despite intervention by his uncle, the connétable de Richemont, the trial for treason and lèse-majesté, overseen by the Attorney General Olivier du Breil, began in Redon on 31 July 1446 in front of the Estates of Brittany.
In 1447, Olivier du Breil refused to hear another trial, while Henry VI of England threatened to intervene militarily to free him.
Poorly treated, he wrote in December to Charles VII of France, who sent the Admiral Prigent de Coëtivy to the Duke of Brittany to demand his release.