In the run-up to the Battle of Poitiers (1356) Guichard seized the fortress of Rochefort and was appointed its castellan by the dauphin.
He participated in the preliminary discussions of the Anglo-French truce at Longjumeau but didn't sign the actual Treaty of Brétigny.
However, Guichard was charged with transferring the fortress of La Rochelle to the English under the terms of the treaty.
He joined the Black Prince's expedition to Spain during Castilian Civil War between 1366 and 1369 and fought in the Battle of Najera of 1367.
In his absence the hostilities resumed, and (according to Froissart) Guichard's embassy had to split and find the way through Savoy, Auvergne and Brittany.