The 37-year-old Gates, who has spent his entire career with the Chargers since they signed him as a free agent out of Kent State in 2003, said at the end of last season that he wanted to play at least one more year.
Considered a lock for the Pro Football Hall of Fame, Gates had a reduced role with the Chargers in 2017, playing 478 snaps and making 30 receptions for 316 receiving yards and three touchdowns.
Gates hadn't had fewer that 49 catches in a season since making 30 as a rookie.
An eight-time Pro Bowl selection and three-time All-Pro, Gates is the Chargers' all-time leader in receptions (927), receiving yards (11,508) and touchdowns (114), with his touchdown total an NFL record for tight ends.
Gates has caught 87 touchdown passes from Philip Rivers, the most in league history for a quarterback-tight end tandem.
He also has 21 games in his career with more than 100 receiving yards, one of only seven tight ends to achieve that feat.