As for Gabby Douglas:
Because she is a London 2012 athlete, including her in the Presidential Delegation would make absolutely no sense to me.
I would think that any London 2012 athlete who stays for the closing ceremony would want to have the special experience and honor of marching in among their fellow athletes and enjoying the festivities as part of that elite group.
President Obama tends to rely on the members of the President's Council for Fitness, Sports, and Nutrition for service of this sort. The official U.S. delegation to the opening ceremonies featured Dominique Dawes (co-chair of the Council, with footballer Drew Brees) and basketball player Grant Hill. The delegation for the closing ceremonies includes baseball player Curtis Pride in addition to Michelle.
There is one member of the Council who is competing in London, sprinter Allyson Felix. (She just won the gold medal in the 200 meters today.)