Even as a young man he entered the Order of the Conventual Friars Minor, becoming a priest and serving multiple times as superior. When in 1453 St. John of Capestrano went to Poland, he joined the Observance. He devoted himself zealously to the apostolic ministry, especially in bringing Russian and Armenian schismatics back to the Church. Kings and princes of Poland petitioned Paul V and Urban VIII for his canonization. Clement XII approved his cult and confirmed the decree of the Sacred Congregation of Rites, which designated G. as one of the principal patrons of Poland and Lithuania.
BIBL.: L. de Clary, Aureola Serafica, III, Quaracchi 1890, p. 95 ff.; Wadding, Annales, XIII, 1932, p. 435; XIV, 1933, pp. 431-36.