The Social Democratic Party said Gross died Thursday but offered no details. Public television said he had been battling illness.
Gross became premier at the age of 34 in July 2004 and took over as chairman of the leftist Social Democrats. He resigned nine months later after weeks of political crisis sparked by a scandal over the financing of his luxury apartment.
He denied wrongdoing but also stepped down as his party chairman and left politics to work for a law firm.
President Milos Zeman, in whose government Gross served as interior minister in 2000-2002 offered his condolences.
After the Sept. 11 attacks in the United States, Gross was the first Czech official to state on record in October 2001 that hijacker Mohamed Atta met with a senior Iraqi intelligence official in Prague in April that year. That purported meeting was cited as evidence of a possible al-Qaida connection to Iraq. The 9-11 commission said that meeting never happened.
Gross is survived by wife Sarka and two daughters.