Peter "Bud" Zuccaro, testifying Wednesday in U.S. District Court in Brooklyn, compared Gotti unfavorably with the tight-lipped mob leaders of the past, the New York Post reported.
"He ruined everything," Zuccaro said. "He publicized everything that was going on. He brought everything that was supposed to be a secret society right out to the forefront, right into the press."
Zuccaro said he originally admired Gotti, the New York Daily News said. By the time Gotti died in a federal prison hospital in 2002, however, Zuccaro said he had changed his mind.
Gotti, nicknamed the "Dapper Don" and "The Teflon Don," became Gambino boss by killing his predecessor, Paul Castellano, in December 1985.
Zuccaro, who has been an FBI informant since 2005, testified at the murder trial of John Burke, a reputed Gambino associate.