Gratzei began his active career as a football player, 1987, at the age of six years, when DSV Leoben. Until 1998, he went through all the youth teams of Donawitzer and was from the 1998 squad in the first team in the second-class First Division. In 2001 Gratzei joined the amateur team of the Grazer AK. He was taken by Franco Foda to the amateurs of SK Sturm Graz in 2002 and as Foda coach of the first team was in the same year, also Gratzei got his chance in the first team mitzutrainieren. On 8 March 2003 Gratzei made his debut in the top division in Austrian football, as he stood at the game against SW Bregenz in goal for the Grazer. After more than half a year due to injury break, he slowly felt his way back to the top approach, but could not due to some error consolidate its position permanently. In the season 2005/ 06 Gratzei was no use.
Since February 2007 he is again the goalkeeper of the SK Sturm. Furthermore belonged Gratzei the extended Austrian squad at the 2008 European Football Championship, but he had, as the third goalkeeper Helge Payer also failed, Ramazan Özcan to defer because he himself also turned out by a knee injury. In autumn 2009 Gratzei was behind goalkeeper Helge Payer second in Austria and made his national team debut on 14 October in the 1-3 defeat against France at the Stade de France, when he replaced Payer in the second half.
On February 1, 2008 Gratzei was first nominated for the squad of the Austrian national team. In April 2008, the club president, coach and manager voted the Bundesliga clubs Gratzei to the goalkeeper of the season 2007/08. In the season 2010/ 11 Christian Gratzei was also elected to the goalkeeper of the season and also was Austrian champion with Sturm Graz.
On 28 January 2011 Gratzei extended his contract expiring at Sturm Graz ahead by four years until season's end of 2014 /15 campaign.