Gruber celebrated his first major success in winter 2004/2005 when he won the overall ranking of the Intercontinental Cup with two victories in Kindberg and Jesenice . In the same year he was at the Junior European Championships 2005 in Kandalakscha tenth in the single seater. In the previous year he was also tenth at the Junior World Championships 2004 in Kindberg. In winter 2007/2008 he was able to repeat his overall victory in the Intercontinental Cup with two wins in Schleis and Deutschnofen .
Gruber has been competing in the World Cup since the 2008/2009 season . In his first winter he achieved four top 10 places, with a seventh place in Deutschnofen being the best result, and thus tenth place overall. He also achieved four top 10 results in the 2009/2010 season . This time he reached three sixth places and at the season finale in Garmisch-Partenkirchen fifth place, which improved him to seventh place in the overall standings. In January 2010, he started at the European Championships in St. Sebastian and achieved tenth place in the single-seater and fourth place in the team competition together with Evelin Lanthaler , Florian Breitenberger and David Mair .
In the 2010/2011 season Gruber only took part in the last World Cup race in Olang , which he finished in seventh position, which meant only 32nd place in the overall World Cup. Before that, he started at the 2011 World Championships in Umhausen , where he finished 13th. In the winter of 2011/2012 Gruber regularly took part in World Cup races again and, in addition to single-seater races, in which two seventh places in Latzfons and Deutschnofen were his best results, he also started in double-seater World Cup races for the first time. Together with Hannes Clara , he placed between sixth and tenth place in the six races of the season, placing him seventh in the overall double-seater World Cup. At the 2012 European Championships in Nowouralsk , Clara / Gruber achieved sixth place in the two-seater and, together with the single-seater Melanie Schwarz and Alex Gruber, fifth place in the team competition, while Stefan Gruber took twelfth place in the single-seater.