The hometown star, who was a staple in the San Diego community, was swapped for some of the Red Sox' top prospects.
Boston and González have been working on a contract extension.
The teams were set to make the deal official Monday morning, sources told the Associated Press.
The Padres, who have a long history of dumping star players because of to financial reasons, will receive minor league right-hander Casey Kelly, first baseman Anthony Rizzo and outfielder Reymond Fuentes, plus a player to be determined later.
Because of the caliber of the prospects they're giving up, the Red Sox wanted to protect themselves by extending Gonzalez's contract beyond 2011. Although the sides couldn't agree by the 2 p.m. EST deadline Sunday, it's thought the sides exchanged parameters and will continue to work toward a deal.
Agent John Boggs has said in recent interviews that González wants fair market value. It's believed that if González were to hit the free agent market, he'd command a contract similar to that of Mark Teixeira, who finished the second season of a $180 million, eight-year contract with the New York Yankees.
Padres general manager Jed Hoyer — who used to work in Boston's front office — has said for some time that the Padres wouldn't be able to afford the kind of big-money contract González would command if he hit the free-agent market.
The Padres recently exercised Gonzalez's $6.2 million option for 2011, the final year of a team-friendly $15 million, five-year contract he signed in 2007.
The left-hander hit .298 with 31 homers and 101 RBIs last season.
In five seasons with San Diego, González has 161 homers — two shy of Nate Colbert's club record — and 501 RBIs. Including parts of two seasons with Texas, he has 168 homers and 525 RBIs.
He set a Padres record with his fourth 30-homer season. His third 100-RBI season tied Phil Nevin for the most in club history.
González had surgery to clean up the labrum in his non-throwing right shoulder on Oct. 20. The Padres said then that the three-time All-Star and two-time Gold Glove first baseman was expected to be ready for spring training.
The popular González, whose parents are Mexican, grew up in the San Diego area and was the No. 1 overall pick by Florida in the 2000 amateur draft. He was obtained in a trade with Texas before the 2006 season and has blossomed into one of the game's best first basemen.
Boston was interested in González last winter.
While many had expected González to be traded before the deadline in July, the Padres were surprise contenders in the NL West and had to keep the slugger.
The Padres went 90-72 despite opening the season with a payroll of $37.8 million, second-lowest in the majors. San Diego was in contention until the final day, when its loss at San Francisco, coupled with Atlanta's win over Philadelphia, eliminated the Padres from the NL West and wild-card races.
The Padres have said they anticipate a payroll of about $40 million.
Based on reporting by the Associated Press.