Gates was signed as an undrafted free agent in 2003 after playing college basketball for the Kent State Golden Flashes.

[3] He ranks seventh in career touchdown receptions, with 116, and leads all tight ends in NFL history.

[10][8] He originally enrolled at Michigan State University wanting to play football under then-coach Nick Saban as well as basketball under coach Tom Izzo.

Recruited by Stan Heath, Gates transferred to Kent State University in northeastern Ohio.

Lost in the disappointment of San Diego's league-worst 4–12 record was Gates' rise from third string to starter.

[22] Gates started off the 2004 season strong with eight receptions for 123 receiving yards in a 27–20 victory over the Houston Texans.

[28] A preferred target of quarterback Drew Brees, Gates finished his second season in the NFL with 81 receptions for 964 yards and 13 touchdowns.

[29] On December 19, Gates tied the NFL single season record for touchdown receptions by a tight end (12) in a 21–0 win over the Cleveland Browns.

[32][33] Gates scored a touchdown in the Wild Card Round overtime loss to the New York Jets.

[34] On August 23, 2005, after holding out for a contract extension, Gates signed a six-year deal worth up to $24 million with the San Diego Chargers.

[39] Gates went on to have another stellar season, catching 89 passes for 1,101 yards and 10 touchdowns, but the Chargers missed the playoffs by a single win.

[42][43] With the departure of Drew Brees during the offseason, it was assumed that Gates would suffer a decline in production in 2006, with first-year starter Philip Rivers at the helm.

Gates had a relatively quiet start to the season, but he finished the year strongly and ended up with 924 receiving yards and nine touchdowns.

[44] He caught two touchdown passes on December 10 in a 48–20 victory the Denver Broncos to help the Chargers clinch their division.

[46][47] Undefeated at Qualcomm during the regular season, the Chargers and their fans anticipated a trip to the Super Bowl but they were beaten by the New England Patriots at home by the score of 24–21 in the Divisional Round.

Gates also pulled out of his fifth Pro Bowl selection[59] because of his injuries alongside his teammate LaDainian Tomlinson.

[60] On February 23, 2008, Gates said he would have foot surgery to fix the injury that had slowed him in the final part of the season.

[80][81] Gates became the Chargers' all-time receptions leader on a six-yard catch on a fourth-and-5, early in the fourth quarter against Detroit.

[82] In Week 6, Gates had two receiving touchdowns in a 35–24 loss to the Denver Broncos, on a night where the Chargers held a 24–0 halftime lead.

[89] Gates passed Lance Alworth to become the Chargers' career leader in receiving yards in Week 7 against the Denver Broncos.

[b] His teammates voted him the Chargers' Offensive Player of the Year[92] and he was invited as an alternate for the Pro Bowl, but declined.

[94] On July 2, 2015, it was announced that Gates would be suspended for the first four games of the 2015 season for violating the NFL's rules on performance enhancing drugs (PEDs).

[95] On October 12, 2015, against the Pittsburgh Steelers in his first game back after from a four-game suspension, Gates scored his 100th and 101st career touchdown receptions.

[101] Following the emergence of second-year tight end Hunter Henry as a starter, Gates only started four games and finished with 30 receptions for a career-low 316 yards and three touchdowns.

[106] In what would be his final NFL game, Gates recorded a receiving touchdown in the 41–28 loss to the New England Patriots in the Divisional Round.

[110] In 2020, Gates announced he was joining the Los Angeles Chargers front office as Legends Ambassador.