The two-time Austrian champion (Sprint 1999, Gundersen 2005) started for the TSU Sankt Veit im Pongau and recorded a total of twenty-six top 10 places in the World Cup . He achieved his best World Cup placements in 2004 with second place in the sprint in Trondheim and in 2003 with third place in the individual competition in Val di Fiemme .
Gruber celebrated his greatest successes at the 2006 Winter Olympics in Turin and at the 2003 Nordic World Ski Championships in Val di Fiemme, where he won the gold medal in the team competition. With the Austrian team he also won the bronze medal at the 2002 Olympic Games in Salt Lake City and the bronze medal at the Nordic World Ski Championships 2005 in Oberstdorf . He was not granted an individual medal at major events, but with fifth place in the sprint competition and sixth place in the individual competition he achieved excellent results at the 2005 World Cup.
On September 27, 2007, Gruber suffered a cruciate ligament rupture while training in Pragelato . After several comeback attempts, which damaged his knee, he announced his retirement from professional sport on April 1, 2008.