As a member of the Golden Flashes, Gates played two seasons as power forward. His junior season he averaged 16.0 points, 8.1 rebounds and 2.7 assists per game. His junior season his team won its first regular season Mid-American Conference (MAC) championship in school history after finishing the regular season with a 24-5 overall record with a 17–1 record in the MAC. Gates helped the Golden Flashes win their second consecutive MAC tournament and earn its resulting berth in the 2002 NCAA Men’s Division I Basketball Tournament In the NCAA tournament he gave crucial performances that helped Kent State reach the Elite Eight as a 10 seed by upsetting 7 seed Oklahoma State (69-61), 2 seed Alabama (71-58) and 3 seed Pittsburgh (78-73).

Gates played all 16 games for the first time since 2009, finishing with 70 receptions (77) for the fourth time in his career. However, Gates only scored four touchdowns, his fewest since his rookie season in 2003.

Lost in the disappointment of San Diego’s league-worst 4–12 record was Gates’s rise from third string to starter. He made his first catch in Week 4 against the Oakland Raiders, scored his first touchdown in Week 10 against the Minnesota Vikings, and enjoyed his first 100-yard receiving game against the Green Bay Packers in Week 15. After finishing a solid 2003 rookie season in which he caught 24 passes for 389 yards and two scores, he was picked by many experts to have a breakout season in 2004. His 16.2 yards per reception were the longest of his career.

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. Because of his holdout, Gates was suspended for one game–the home opener against the Dallas Cowboys, a loss. 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.

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. He caught two touchdown passes on December 10 against the Denver Broncos to help the Chargers clinch their division.

In 2007, Gates was not selected to the All Pro team but was selected to the Pro Bowl. During the season, he caught 75 passes for 984 yards and nine touchdowns. He was once again a key factor in the Chargers 11-5 campaign but not during the playoffs. He dislocated a toe on his right foot just before the postseason, which slowed him down during the Chargers playoff run.

On February 23, 2008, Gates said he would have foot surgery to fix the injury that has hurt him in the final part of the season. He would face a 4–6 month recovery time. He seemed questionable for Week 1 and even sounded worried about his status for the opening day of the 2008 season, however, he ended up playing every game despite complaints that his toe was bothering him during the first half of the season. He finished the season with 60 receptions for 704 yards and eight touchdowns.

His jersey number, 44, was retired on February 27, 2010, making him just the fourth Golden Flash to receive the honor.

Gates married his longtime girlfriend, model Sasha Dindayal, on July 9, 2011. As of 2014, the couple resided in Encino, California.

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). On October 12, 2015, in his first game back after from a four-game suspension, Gates scored his 100th and 101st career touchdown receptions. He would finish the season with 630 yards and 5 touchdowns on 56 receptions. On March 9, 2016, Gates signed a 2-year contract extension to remain with the Chargers.

Gates played in 14 games in 2016, starting 9. He caught 53 passes for 538 yards, and scored 7 touchdowns. His 10.3 yards per reception and 57% completion percentage were the lowest of his career.

On April 27, 2018, the Chargers informed Gates they would not be bringing him back for the 2018 season.

On September 2, 2018, Gates re-signed with the Los Angeles Chargers following an injury to Hunter Henry. In Week 4, against the San Francisco 49ers, he scored his first receiving touchdown of the season.

Gates announced his retirement on January 14, 2020. He ended his career with a record 116 touchdown catches by a tight end, including 89 thrown by Rivers for the most ever by a quarterback–tight end duo. He also finished as the Chargers’ franchise leader in receptions (955) and receiving yards (11,841).