Gandolfini had appeared in advertisements for Kobold's company, Kobold Watches.
Gandolfini was pronounced dead at 11 p.m. Wednesday in Rome after being taken by ambulance to the Policlinic Umberto I hospital.
Dr. Claudio Modini, head of the hospital emergency room, said Gandolfini arrived at the hospital at 10:40 p.m. (2040 GMT; 4:40 p.m. EDT) and was pronounced dead after resuscitation efforts in the ambulance and hospital failed.
An autopsy would be performed starting 24 hours after the death, as required by law, Modini told The Associated Press.
The actor, known for his portrayal of the tortured Italian-American mob boss Tony Soprano, was to have received an award and taken part in the closing ceremony Saturday of the Taormina Film Festival, which takes place against the backdrop of Taormina's spectacular Roman amphitheater.
He also was to have given a special class Saturday morning at the festival, as was done by actor Jeremy Irons earlier in the week.
Festival organizers Mario Sesti and Tiziana Rocca said instead they would organize a tribute "to celebrate his great achievement and talent." They said they had heard from Gandolfini a few hours before he died, and "he was very happy to receive this award and be able to travel to Italy."
"The Sopranos" was a hit when it first aired in Italy in 2001, with critics giving it rave reviews, despite some criticism from Italian-Americans across the Atlantic who thought it stereotyped them.
"Rarely does one see fiction so intelligent, ironic, full of psychological and narrative subtleties. And the dialogue! The photography!" Italy's most prominent TV critic, Aldo Grasso, gushed in the leading daily Corriere della Sera after the first episode aired. "Trust me, don't miss 'The Sopranos!'"
The daily La Repubblica called the show a "masterpiece." The paper deplored that it was being "hidden" from viewers by being aired at the unenviable 12:30 a.m. timeslot and urged it to be moved up - something former Premier Silvio Berlusconi's Mediaset network eventually did, showing it at 11:30 p.m.
Gandolfini's death was one of the top news stories in Italy on Thursday, with American tourists outside his hotel well aware of the tragedy.
"I thought he was a great actor," said William Capece, visiting Rome from Houston, Texas. "Pretty sad because it is a big loss to the field of acting."
The U.S. Embassy in Rome, which said it had learned about the death from the media, said it would be available to provide a death certificate and help prepare the body for return to the United States. The embassy said it can often take between four and seven days to arrange for it to be sent outside of Italy.
The embassy spokesman declined further comment, directing inquiries to the family.
It isn't yet known yet what caused his heart to stop beating. Sudden cardiac arrest can be due to a heart attack, a heart rhythm problem, or a result of trauma. The chance of cardiac arrest increases as people get older; men over age 45 have a greater risk. Men in general are up to three times more likely to have a sudden cardiac arrest than women.
Friend: Stricken Gandolfini found by family member
A friend of "Sopranos" star James Gandolfini said Thursday the actor was discovered by a family member in his hotel room in Rome before he was pronounced dead of cardiac arrest at a hospital.
Michael Kobold, who described himself as a close family friend, read a short statement to reporters, but said little more about the circumstances of Gandolfini's death on Wednesday night.
He did not say who discovered Gandolfini, 51, but NBC quoted Antonio D'Amore, manager of the Boscolo Excedra hotel, as saying it was the actor's 13-year-old son, Michael.
Gandolfini had appeared in advertisements for Kobold's company, Kobold Watches. Stars share reaction to James Gandolfini's death
James Gandolfini's fans and colleagues shared reaction to his death Wednesday:
"I am shocked and devastated by Jim's passing. He was a man of tremendous depth and sensitivity, with a kindness and generosity beyond words. I consider myself very lucky to have spent 10 years as his close colleague. My heart goes out to his family. As those of us in his pretend one hold on to the memories of our intense and beautiful time together. The love between Tony and Carmela was one of the greatest I've ever known." Edie Falco., who played Tony Soprano's wife, Carmela, on "The Sopranos."
"Jimmy treated us all like family with a generosity, loyalty and compassion that is rare in this world. ... Working with him was a pleasure and a privilege. I will be forever grateful having had a friend the likes of Jimmy." Michael Imperioli.
"He was a genius. 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. A great deal of that genius resided in those sad eyes. He was my partner. ... He was my brother in ways I can't explain and never will be able to explain." David Chase, creator of "The Sopranos."
"My heart goes out to his family. James was incredibly talented and I feel very fortunate to have had the chance to work with him. He was very committed during the shooting of 'True Romance.' I remember Tony Scott saying he slept in his suit in his car to stay in character. His work as Tony Soprano was flawless. It is a real loss to the creative community." Patricia Arquette.
"James Gandolfini was a friend and a great actor. He wrote me after portraying me last year, which was a great thrill and honor. I told him I was glad an Italian played me swear words and all. We laughed together at the fact that tough guys can have a heart of gold. He did, and we will miss him." Leon Panetta. Gandolfini played then-CIA director Panetta in Kathryn Bigelow's Osama bin Laden hunt docudrama "Zero Dark Thirty."
"If Broadway has a version of a guy you want in your foxhole, Jim Gandolfini was mine. During our time together in 'God of Carnage,' we played 320 performances together. He didn't miss one. Sadly, I now miss him like a brother." Jeff Daniels.
"The great James Gandolfini passed away today. Only 51. I can't believe it." Bette Midler on Twitter.
"It's an awful shock. James Gandolfini was a fine actor, a Rutgers alum and a true Jersey guy. I was a huge fan of his and the character he played so authentically, Tony Soprano. I have gotten to know Jimmy and many of the other actors in the 'Sopranos' cast and I can say that each of them are an individual New Jersey treasure." New Jersey Gov. Chris Christie.
"We lost a giant today. I am utterly heartbroken." Lorraine Bracco.
"James Gandolfini what a nice guy, so funny and sweet. He will be sorely missed. Our deepest condolences to his family." Aerosmith guitarist Joe Perry on Twitter.
"I am heartbroken about James Gandolfini. He was a gentle giant and great man. I love this picture, I loved him." Actress Rose McGowan, who posted a snapshot of her and Gandolfini on Twitter.
"My condolences to the family and all those who loved James Gandolfini a true NJ Great and NJ Original. RIP." Newark Mayor Cory Booker on Twitter.
"Jimmy was one of the most talented, authentic and vulnerable actors of our time. He was unorthodox and truly special in so many ways. He had the sex appeal of Steve McQueen or Brando in his prime as well as the comedic genius of Jackie Gleason." Brad Grey, a producer of "The Sopranos."
"You will be missed greatly James Gandolfini Maybe best actor ever on a tv series. Salute'" Comedian Artie Lange on Twitter.
"awful awful news. James Gandolfini will be missed. He was a great actor. Just great." Mia Farrow on Twitter.
"James Gandolfini. Unbelievably sad news. A fine man." Steve Carell on Twitter.
"My thoughts and prayers go out to James Gandolfini's family. An extraordinary actor. RIP, Mr. Gandolfini." Robin Williams on Twitter..
"Fifty-one and leaves a kid he was newly married. His son is fatherless now ... It's way too young." Joe Gannascoli, who played Vito Spatafore on "The Sopranos."
"James Gandolfini.. So talented, so nice and far too young to be taken away. You will be missed Sir" Actress Shannen Doherty on Twitter.
"I have lost a brother and a best friend. The world has lost one of the greatest actors of all time." Musician Steven Van Zandt.
"R.I.P. James Gandolfini. Such a great actor. More than just tony soprano and that would have been a lot on its own." Musician Rob Thomas on Twitter.
"I am so deeply saddened by the untimely passing of sweet James Gandolfini! My heart is broken for his family!! I'm in shock worked with him in "Surviving Christmas"" Christina Applegate on Twitter.
"James Gandolfini was a kind, funny, wonderful guy. I'm so lucky to have worked with him. Sending love to his family. Such a sad, sad day. worked with him on "The Incredible Burt Wonderstone"" Olivia Wilde on Twitter.
"The Rutgers University community mourns the sad and sudden loss of James Gandolfini, a 1983 Rutgers graduate who was inducted into the Rutgers Hall of Distinguished Alumni in 2004. Mr. Gandolfini was a proud and passionate supporter of the university for many years. Our thoughts and prayers go out to Mr. Gandolfini's family." Statement from Rutgers University in New Jersey.
"We came in honor of him. I've never been here, so this was a way to honor him." North Arlington, N.J., resident Sheryl Martin, who with her husband was visiting nearby Holsten's, the restaurant where the last episode of "The Sopranos" was shot.