Gassner won the Suzuki Rallye Cup in Germany in 2007, finishing seventh overall in the German Rally Championship and winning Division 4. He made his
World Rally Championship debut on
Rally Deutschland, which was also a round of the German championship. In 2008, he finished fourth in the German championship in a
Mitsubishi Lancer Evo IX. He was also the runner-up in the
Mitropa Rally Cup behind his father. He contested the German and British rounds of the WRC, finishing 27th and 20th respectively. In 2009, Gassner won the
German Rally Championship with four wins, as well as the Mitropa Rally Cup. He also entered five rounds of the WRC, with a best result of 12th overall in
Portugal. In 2010, Gassner entered six rounds of the WRC in an Evo IX. He finished fourth in the
Production World Rally Championship category in
Germany, and also finished 13th overall and first in
Group N in
Spain. For 2011, Gassner joined the
Red Bull Škoda team in the SWRC, finishing third in the category in
Jordan, fifth in
Sardinia and fourth in
Greece. ==World Rally Championship results==