Please answer a question about the following article about Middle Ages:

Chivalry and the ethos of courtly love developed in royal and noble courts. This culture was expressed in the vernacular languages rather than Latin, and comprised poems, stories, legends, and popular songs spread by troubadours, or wandering minstrels. Often the stories were written down in the chansons de geste, or "songs of great deeds", such as The Song of Roland or The Song of Hildebrand. Secular and religious histories were also produced. Geoffrey of Monmouth (d. c. 1155) composed his Historia Regum Britanniae, a collection of stories and legends about Arthur. Other works were more clearly history, such as Otto von Freising's (d. 1158) Gesta Friderici Imperatoris detailing the deeds of Emperor Frederick Barbarossa, or William of Malmesbury's (d. c. 1143) Gesta Regum on the kings of England.

Who was the subject of Gesta Friderici Imperatoris?
Emperor Frederick Barbarossa