Son of Peter II of Gabarret and Guiscarda of Béarn, he received the vicecomital title after the death of his maternal uncle Centule VI.
Peter participated in the "crusade" of 1148 against the Moors in Spain organised by Raymond Berengar IV of Barcelona, his wife's cousin.
That final conquest had symbolic significance for the Bearnese, as Centule VI had died at Fraga's walls.
Peter also promoted the pilgrimage practice, Way of Saint James and, contrary to his grandfather Gaston IV, established hospitals along the route through the pass of Roncesvalles and not that of Somport.
His final public act was the foundation of a new pilgrim hospital in the year of his death, which shortly followed.