After speaking with Raiders owner Al Davis, Gallery has decided to sign somewhere else whenever free agency begins.
Oakland made Gallery the second overall pick in 2004. He started 91 games for the team.
"With the end of the league year coming today, and the few conversations I've had with the Raiders and Mr. Davis, we've come to a mutual agreement that it's time for me to move on with my career," Gallery said Thursday from his home in the Oakland area. "I appreciate the start that they and Mr. Davis gave me and I truly have enjoyed my time wearing the silver and black. I wish them well, I thank the fans and I take away great memories of my time in Oakland."
Raiders senior executive John Herrera told The Associated Press on Thursday the decision was made because of the vast difference in contract proposals. Herrera says Gallery was seeking about $8 million a season while Oakland was willing to offer about $2.5 million.
Rick Smith, Gallery's agent, says it was clear from talking to the Raiders they want to give some of their young players a chance.
Like Asomugha, Gallery's contract voided after this past season. After seven NFL seasons, he will be an unrestricted free agent. The 30-year-old Gallery knows that whenever football is played next, he will be playing it in a different place.
"I'm looking for a new start somewhere else, to experience another place, and to be able to accomplish some of the things I want to," Gallery said. "It's best for everyone."
Gallery is more comfortable playing left guard, though he could play other positions along the offensive line. To try to replace Gallery, the Raiders are expected to turn to last year's fourth-round pick, Bruce Campbell, another favorite of Davis.
But each side will go its own way as Oakland will be losing another key player who helped make the Raiders respectable and relevant last season.
Although Gallery and Asomugha are set to leave Oakland, the Raiders have been one of the more aggressive teams in re-signing players this offseason.
Along with signing linebacker Kamerion Wimbley to his franchise tender at approximately $11.3 million, Oakland gave cornerback Stanford Routt a $31.5 million, three-year deal; handed defensive tackle Richard Seymour a $30 million, two-year contract with $22.5 million in guarantees; signed defensive tackle John Henderson to an $8 million, two-year deal; and signed special-teams standout and backup running back Rock Cartwright to an undisclosed contract.
"I'll always remember being able to come out of the tunnel of that stadium, into the Black Hole," Gallery said. "That was exhilarating and never got old. I loved being a part of a group that helped bring the Raiders back to where they were in the past. It's just that now it's time to go someplace else."
Adam Schefter is ESPN's NFL Insider. Information from The Associated Press was used in this report.