New Jersey Gov. Chris Christie was among them, saying the news was "an awful shock."
Heart attacks can happen at any age. But usually, first heart attacks don't strike until the victim is older than 60.
"This is young to have a heart attack, even when you look at any pre-existing health conditions," CNN chief medical correspondent Sanjay Gupta said.
However, risk factors such as obesity, smoking, lack of exercise and stress can cause them to occur earlier in life, Gupta said.
"He loved his food. He didn't push back from the table," said journalist Larry King, who spent time with the actor on and off camera.
Photos and anecdotes indicate that Gandolfini enjoyed cigars.
Though intense outbursts of adrenaline-infused anger were a mainstay of Tony Soprano's character, he was a far cry from Gandolfini.
Jovial, fun, reserved
Colleagues described him as funny, kind and gentle. He had a reputation for often being reserved and quiet. He didn't seek publicity, avoiding media coverage, journalists have said.
Edie Falco, the actress who played Tony's wife, Carmela, said Gandolfini 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," Falco said in a statement Thursday. "The love between Tony and Carmela was one of the greatest I've ever known."
Former CIA chief Leon Panetta, played by Gandolfini in the movie "Zero Dark Thirty," also issued a statement about the actor Thursday.
"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."
Martin Dempsey, chairman of the Joint Chiefs of Staff, also mourned the "Sopranos" actor.
"Yesterday, we lost a talented actor and advocate for our service members. Our thoughts and prayers are with the Gandolfini family," a statement from the general said Thursday.
Tributes poured in on social media, describing Gandolfini as jovial and likable.
Actor Mark Ruffalo shared his grief on Twitter: "Oh Jimmy. It's a crying shame. You stormed in and out. Your voice like velvet granite. From a whisper to a shout. A comet of a man."
'He lit up the room with a big smile'
Even after his "Sopranos" fame, he was very down to earth, said iReporter Pat Tantalo, who worked on the set of a 2006 movie Gandolfini starred in.
He lit up the room with a big smile the first time he walked into the production office, Tantalo recalled.
"We instantly connected. He planted his huge mitts on my shoulders and called me a little bull."
Tantalo posted a goodbye tribute on CNN's iReport website with a photo of Gandolfini's arm around his shoulder. Both men are smiling ear to ear.
They exchanged telephone numbers and spent some of their evenings sitting around together on a deck by a lake smoking cigars and telling stories.
"It was like we had been friends forever," Tantalo said.
IReporter Shana O'Neil met Gandolfini when he was shooting in her office in 1994. She remembers him as "Jersey through and through," a trait he did share with mobster Tony Soprano.