Gaven became (at the time, since passed by Freddy Adu) the youngest player to sign with MLS. At the age of 16 years and two months, Gaven was drafted by the MetroStars with the twelfth overall pick in the 2003 MLS SuperDraft. He became the youngest player in team history on his debut on June 14, 2003 against Chicago and became the youngest player in team history to score a goal, which he did in just his second game, against D.C. United on July 5. With the game tied at two, Metro coach Bob Bradley used a loophole in MLS’s substitution rules, which allowed a fourth substitution in a game but only for a goalkeeper. Bradley first exchanged the playing positions of Metro goalie Tim Howard and midfielder Mark Lisi, then substituted Gaven into goal for Lisi as the fourth “goalkeeper only” sub. After only 10 seconds of play, the ball went out of play and Gaven and Howard switched places, with Gaven moving to midfield and Howard back into goal. Gaven then went on to score the game-winner in overtime. In the process, Gaven became the youngest goalkeeper in MLS history. The goalkeeper fourth-substitution rule was eliminated after the season, largely due to the outcry following Gaven’s goal.

Gaven played every minute for the U.S. in the 2003 Under-17 World Championship in Finland. He then became the youngest player (17 years 104 days) in CONCACAF history to appear in an Olympic qualifier when he played for the U.S. Under-23 team in a 2-0 victory over Canada on February 5, 2004. He then became the fourth youngest player in US history to get his a senior cap, doing so in a 1-1 against Poland on July 11, 2004.

After becoming an automatic starter and coming into his own with the MetroStars in 2004, Gaven became the youngest player ever voted in to start the MLS All-Star Game, finished the MLS season with seven goals and seven assists, and became the youngest player in league history to be named to the MLS Best XI.

In 2005, after continuing his solid play with the MetroStars, Gaven was part of the U.S. Under-20 team at the 2005 FIFA World Youth Championship in the Netherlands.

Gaven ended the 2005 MLS season with eight goals and four assists. Just before the 2006 season, Gaven was traded to Columbus Crew along with the rights to defender Chris Leitch for striker Edson Buddle. With the Crew, Gaven played nearly every game in 2006, scoring four goals. In 2007, he appeared in 27 games for Columbus scoring five goals and tallying seven assists. Gaven’s 2008 season resulted in lower output, three goals and two assists, due in part to starting the season injured. In the Eastern Conference Championship game, Gaven propelled the Crew into their first MLS Cup Final with a game-winning goal; the Crew went on to win the Final.

During the 2010 MLS season in a game against the Philadelphia Union, Gaven was forced to play in goal for the second time in his MLS career after goalkeeper William Hesmer went out of the game with a shoulder injury. The Crew had used all three substitutions prior to Hesmer’s injury. Gaven was scored on by Sebastian LeToux during the final minutes of the game, but made two saves to help the Crew win the game 3–1. Gaven would go on to win MLS Save of the Week honors for one of the saves.

Gaven signed a contract extension for two years plus option years with Columbus on November 14, 2011.

Following his retirement from soccer in 2013, he moved to Ave Maria University as Head Men’s Soccer Coach in March 2016.