James Hester, a longtime friend, said he found Simmons' body Thursday in the room Simmons rented in New York. Cause of death has not been determined but Hester said Simmons was hospitalized with pneumonia in November and December 2013.
Simmons, nicknamed "Sugar Bear," played for the Giants from 1979 to 1983 and for the Washington Redskins in 1983.
He kept his homosexuality a secret until he publicly came out on a television program in 1992.
He said he had no choice, telling an interviewer in 2006: "In the NFL, there is nothing worse than being gay. You can beat your wife but you better not be gay."
Former teammate Harry Carson told the New York Daily News Monday: "All the guys in the locker room loved him. We would have accepted him (as openly gay). Nothing would have happened to him."
Simmons moved to San Francisco after his retirement, becoming a homeless prostitute and a drag queen, the newspaper said. He appeared on a religious television show in 2005 and told evangelist Pat Robertson homosexuality is against God's will.
Simmons is survived by a daughter, a grandson and five siblings. Hester said his family is raising money to return Simmons' body to his hometown of Savannah, Ga.