An Ethiopian runner won the race for the second time since women began running the marathon at the Olympics 18 years ago.
The race drew 118 runners, who competed in the rain on the streets of central London.
Gelana was part of a large leading group as the runners neared the point where water bottles were available on long tables alongside the road. Like most of the others, she veered toward those tables to grab a bottle on the run and as she did so she was caught up in the inevitable congestion and hit the ground.
She quickly got to her feet and although she was grimacing in pain she did not lose contact with the frontrunners.
A group of four runners was left to challenge for the medals over the final mile and when Gelana picked up the pace they fell away one by one.
She crossed the line in 2 hours, 23 minutes, 7 seconds and finished 5 seconds in front of Priscah Jeptoo of Kenya. Tatyana Petrova Arkhipova of Russia was third, 17 seconds behind Jeptoo.
Pre-race favorite Mary Jepkosgei Keitany of Kenya was fourth, followed by Tetyana Gamera-Shmyrko of Ukraine, Zhu Xiaolin of China, Jessica Augusto of Portugal, Valeria Straneo of Italy, Albina Mayorova of Russia and Shalane Flanagan of the United States.