Guglielminpietro's club career reached its peak at the Italian club A.C. Milan, where he shared the limelight with some of the game's most notable stars, such as Paolo Maldini, Demetrio Albertini, Roberto Donadoni, Alessandro Costacurta, Leonardo, George Weah and Oliver Bierhoff. On the final matchday of the season, Guly scored the opening goal in a 2–1 away win against Perugia that won Milan the 1998–99 "Scudetto". His play declined during the following seasons due to injuries, and he was transferred to different clubs around the globe, never recovering his top ability. His first port of call after leaving Milan was at rivals Internazionale whom he joined in 2001. He scored his first and what turned out to be his only goal for the club in a UEFA Cup tie against FC Brașov on 27 September 2001.