He assumed the title at thirteen, under his mother's regency. The Duchy soon disintegrated: among the various parties contending its lands, the condottieroFacino Cane prevailed. Taking advantage of Gian Maria's cruelty, he managed to create in him doubts about Caterina, who was imprisoned in Monza, where she died on 17 October 1404, probably murdered. The duke was famous for his dogs, which were trained to slaughter men.
A plot by a party of MilaneseGhibellines was raised against the Duke when Facino Cane was ill in Pavia, and Gian Maria was assassinated in front of the church of San Gottardo in Milan. The dying Facino had his officers swear to support Filippo Maria, Gian Maria's brother, who in fact succeeded him.