Apparently of a heart attack while in Italy. He was 51. Here’s a bit from The New York Times’ obituary:
James Joseph Gandolfini Jr. was born in Westwood, N.J., on Sept. 18, 1961. His father was an Italian immigrant who held a number of jobs, including janitor, bricklayer and mason. His mother, Santa, was a high school lunch lady.
He attended Park Ridge High School and Rutgers University, graduating in 1983 with a degree in communications. He drove a delivery truck, managed nightclubs and tended bar in Manhattan before becoming interested in acting at age 25, when a friend brought him to an acting class. [...]
Survivors include his wife, Deborah Lin; a daughter, Liliana, born last year; and a teenage son, Michael, from his marriage to Marcella Wudarski, which ended in divorce.