Gandolfini and his family were visiting Rome prior to his scheduled appearance on Saturday as guest of honor at the Taormina Film Festival in Sicily.
"He was a genius," said "Sopranos" creator David Chase. "Anyone who saw him even in the smallest of his performances knows that. He is one of the greatest actors of this or any time."
Edie Falco who played Carmela Soprano, the unforgettable mobster's wife, said Gandolfini was "a man of tremendous depth and sensitivity, with a kindness and generosity beyond words."
"My heart goes out to his family," she added.
Jamie Lynn Sigler spent 10 years playing Gandolfini's daughter on the HBO ground-breaking series and said she was "heartbroken" to hear of his passing.
"I spent 10 years of my life studying and admiring one of the most brilliant actors , yes, but more importantly one of the greatest men," she said. "Jim had the ability, unbeknownst to him, to make you feel like everything would be alright if he was around. I treasure my memories with him and feel so honored that I was an up close witness to his greatness."
Michael Imperioli, who played Tony Soprano's nephew, called working with Gandolfini a "pleasure and a privilege" in a statement. "Jimmy treated us all like family with a generosity, loyalty and compassion that is rare in this world...I will be forever grateful having had a friend the likes of Jimmy."
Gandolfini won critical acclaim, three Emmy Awards and three Screen Actors Guild awards and a Golden Globe for playing Tony Soprano from 1999-2007.
(Partial excerpts from NBC news added in.)