On 11 August 2013, Cardiff City confirmed that they had signed Medel from Sevilla for a club record £11 million fee on a four-year deal. He made his debut for the club on 17 August, in a 2-0 defeat against West Ham United. He made his home debut at the Cardiff City Stadium on 25 August, in a 3-2 win against Manchester City. Medel played all 90 minutes and received the fans man of the match award.
He made his international debut for Chile, in March 2007 in a friendly against Argentina. Later that year, he played a key role on the third place for Chile in the U-20 World Cup held in Canada. However, he didn't play the 3rd place match because he was sent off in the semifinal match against Argentina, when he kicked Gabriel Mercado in the 24th minute. He made his official debut for Chile playing in the 2010 FIFA World Cup qualification against Bolivia in a 0-2 victory, scoring both goals, the first being an spectacular bicycle kick. Gary played an important role on Chilean qualification to 2010 FIFA World Cup, playing as a central defender or right back. He played in the 2010 World Cup, starting in the three group-stage matches, but being suspended for the Round of 16 match against Brazil. In 2011 he played in the Copa América, being sent off in the Quarter-final match against Venezuela, after making an intentional handball. He played a key role in 2014 FIFA World Cup qualification, scoring the goal that secured qualification for the 2014 FIFA World Cup, a heading goal against Ecuador. Medel skippered his national side on a friendly against Germany on March 5th, 2014, which Chile lost 1-0.