The eight-year NFL veteran determined his body wouldn't be able to handle training camp and a full season, a source close to the player said.
The source explained that Gallery did his best to get himself into shape before training camp, but he realized his body couldn't get through it.
There wasn't one injury that pushed Gallery to retire, the person close to the player said, adding that he had considered calling it quits for the past four or five days.
Gallery, who turned 32 last month, signed with the Patriots in March.
Left guard Logan Mankins is still rehabilitating from a torn anterior cruciate ligament, and right guard Brian Waters has yet to report to training camp. In April, 11-year veteran left tackle Matt Light retired.
Gallery had surgery to repair a torn groin muscle last season.
Gallery was the No. 2 overall draft pick by the Raiders in 2004. He was drafted as a tackle but moved to guard in 2007. He played seven seasons with the Raiders before signing with the Seahawks for the 2011 season.
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