ESPN first reported the developmenet.
Gates, who turns 38 on June 18, wants to continue playing, but he will need to find another team willing to sign an aging veteran.
Once one of the NFL's most-feared tight ends, Gates' production took a nosedive in 2017. He appeared in 16 games with just four starts as Hunter Henry emerged as the go-to option.
Gates totaled 500 offensive snaps compared to Henry's 598. Henry, who started 13 games, had 45 catches for 579 yards and four touchdowns on 62 targets. Gates was targeted 52 times for 316 yards receiving -- the lowest total of his 15-year career -- and had three touchdowns on 30 catches.
The 6-foot-4, 255-pound Gates, a former college basketball player, entered the league with the Chargers in 2003 as an undrafted free agent out of Kent State.