At the 2012 U.S. Olympic Trials in Omaha, Nebraska, the U.S. qualifying meet for the 2012 Olympics, Gemmell made the U.S. Olympic team for the first time by finishing first in the 1,500-meter freestyle with a time of 14:52.19. At the 2012 Summer Olympics in London, he placed ninth in the preliminary heats of the 1,500-meter freestyle with a time of 14:59.05, and did not advance among the top eight to the event final.