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.