Webb was born in Corona, California. His father was a Marine sergeant, and the family moved frequently, as his career took him to new assignments.

Gary found work in 1978 as a reporter at the Kentucky Post, a local paper affiliated with the larger Cincinnati Post.

In 1979, Webb married Susan Bell; the couple eventually had three children.

Webb's first major investigative work appeared in 1980, when the Cincinnati Post published "The Coal Connection," a seventeen-part series by Webb and Post reporter Thomas Scheffey.

In 1983, Webb moved to the Cleveland Plain Dealer, where he continued doing investigative work.

A 1985 series, "Doctoring the Truth," uncovered problems in the State Medical Board and led to an Ohio House investigation which resulted in major revisions to the state Medical Practice Act

In 1988, Webb was recruited by the San Jose Mercury News, which was looking for an investigative reporter. He was assigned to its Sacramento bureau, where he was allowed to choose most of his own stories.

Webb began researching "Dark Alliance" in July 1995.

The series was published in The Mercury News in three parts, from August 18–20, 1996, with one long article and one or two shorter articles appearing each day. It was also posted on The Mercury News website with additional information, including documents cited in the series and audio recordings of people quoted in the articles.

Webb resigned from the paper in November 1997.

Webb expanded the "Dark Alliance" series into a book that responded to the criticism of the series and described his experiences writing the story and dealing with the controversy. It was published in 1998 as Dark Alliance: The CIA, the Contras, and the Crack Cocaine Explosion.

Webb was found dead in his Carmichael home on December 10, 2004, with two gunshot wounds to the head.

Kill the Messenger is a 2014 American mystery film directed by Michael Cuesta.The film stars Jeremy Renner, Ray Liotta, Barry Pepper, Andy Garcia, Mary Elizabeth Winstead, Michael Sheen, Oliver Platt, Tim Blake Nelson, Robert Patrick, Michael Kenneth Williams, and Paz Vega, and the movie is scheduled to be released in all theaters starting October 17, 2014. Production companies are Bluegrass Films and The Combine