At the 2008 FINA Youth World Swimming Championships, Gemmell won gold in the 400 m individual medley.
At the 2009 World Swimming Championships, Gemmell won silver in the men's 10 km marathon swim. In the 5 km race, Gemmell finished 5th. Gemmell is a recipient of the United States 2009 male open water swimmer of the year award and was nominated for the 2009 Golden Goggle Awards as breakout swimmer of the year. At the 2010 Pan Pacific Swimming Championships, Gemmell placed 5th in the 1500 m freestyle and 11th in the 400 m individual medley.
He participated in the 2011 FINA World Championships in Shanghai, China where he was part of the gold medal winning 5 km Team Pursuit team. He also won the 2012 USA Swimming 10km Open Water Swimming Championships in Ft. Myers, Florida. He also won the men's 10 km race at the 2012 USA Swimming National Open Water Swimming Championships in Fort Myers, Florida.