Gian Galeazzo died in 1494 at Palazzo Ducale, the summer home of the Visconti and Sforza families.
During that time, he received a visit from Charles VIII of France.
According to the Italian historian Francesco Guicciardini in his The History of Italy (Italian: La Historia di Italia), he was poisoned by his uncle, Ludovico il Moro.

The rumor was widespread that Giovan Galeazzo's death had been provoked by immoderate coitus; nevertheless, it was widely believed throughout Italy that he had died not through natural illness nor as a result of incontinence, but had been poisoned … one of the royal physicians … asserted that he had seen manifest signs of it.
Nor was there anyone who doubted that if it had been poison, it had been administered through his uncle Ludovico Sforza's machinations …
Gian Galeazzo died in 1494 at Palazzo Ducale, the summer home of the Visconti and Sforza families. During that time, he received a visit from Charles VIII of France. According to the Italian historian Francesco Guicciardini in his The History of Italy (Italian: La Historia di Italia), he was poisoned by his uncle, Ludovico il Moro.
The rumor was widespread that Giovan Galeazzo's death had been provoked by immoderate coitus; nevertheless, it was widely believed throughout Italy that he had died not through natural illness nor as a result of incontinence, but had been poisoned … one of the royal physicians … asserted that he had seen manifest signs of it. Nor was there anyone who doubted that if it had been poison, it had been administered through his uncle Ludovico Sforza's machinations …
Gian Galeazzo Sforza