Gian Galeazzo is in a much better situation than Venice would be 100 years later: the great Western schism is in full swing, and the pope is in Avignon; Venice has not yet discovered a true appetite for the Padan plains, and her gaze is concentrated on the east.
Considering the "distraction" of France and the parlous state of the HRE, GG has a true window of opportunity, which he's already exploited by gaining the lordship of most of Northern Italian cities and a goodly number of the Central Italian cities (Bologna has just submitted, and at the time of his death Florence is negotiating a surrender).
IMHO, the big challenge that confronts GG is to hammer together a string of lordships to forge a unitary kingdom (and maybe the time is also right for him to purchase the crown of Italy from the HRE), as well as continue to keep in check his mercenary condottieri (and possibly get rid of a bunch of them).
When he dies IOTL he's around 50 years of age: living another 10-15 years should not be impossible, barring the risks inherent to his position.
He's not even to get into a lot of wars: his task is almost complete, and he can concentrate on government, and raising his two sons in a way that the succession is assured, and he'll be followed by another good ruler. No reason (and no benefit) in trying to pick up a fight with Venice: the 14th century Venice is not a true competitor for mainland, he can prosper on the traffics that will move toward central Europe (and he's in control of both the Brenner road and the Valtellina road) and in any case he cannot attack Venice itself.
Give GG and his successor (looks like his younger son is more promising than the elder) 30 years and the unification of Italy is a fait accompli.