Gay dipped across the line in 9.99 seconds, 0.04 ahead of Gatlin. Lemaitre clocked 10.08.
Gatlin, the 2004 Olympic champion, held off Gay last month to win the 100 at the US Olympic trials.
In other events at the Diamond League event, Javier Culson of Puerto Rico in the men’s 400 hurdles, Kenyan runner David Rudisha in the 800 and Dejen Gebremeskel of Ethiopia in the 5,000 set world-leading times this year.
Kenenisa Bekele conceded he would not defend his 5,000 metres Olympic title when he was only the fifth Ethiopian to cross the line.
Bekele, who has suffered calf problems over the past three years, had to finish among the top three Ethiopians but finished way down the field, crossing the line in ninth place overall and will now only run the 10,000 in London.
World champion Dai Greene dispelled any doubts about his form and fitness with an impressive personal best, but a seriously fast 5,000 metres race showed just how tough a challenge Mo Farah will face at the Olympics.
Greene clocked 47.84 seconds, just 0.02secs outside Kriss Akabusi’s 20-year-old British record, although it was only good enough for second place behind Javier Culson.
The Puerto Rican, who took silver behind the Welshman at the World Championships last year but is firmly the world No 1 this, won in a world-leading time of 47.78s to further enhance his claims for gold in London in a month’s time.
Farah was not running, but the 5,000m saw ten athletes go quicker than his previously world-leading time of 12:56.98. Ethiopia’s Dejen Gebremeskel won in 12:46.81 to become the fifth fastest of all time over the distance, while it was the quickest time since 2005. His fellow countryman Hagos Gebrhiwet was second in a world junior record of 12:47.53 as the first six home all went quicker than Farah has ever gone. World and Olympic champion Kenenisa Bekele, another Ethiopian, ran a season’s best 12:55.79, but that was only good enough for ninth, meaning he is unlikely to run over the distance at the Olympics.
Australia’s Sally Pearson looks another athlete nailed on for gold in London after winning the 100m hurdles in a world-leading 12.40s, with Britain’s Tiffany Porter finishing third.
Christine Ohuruogu ran a season’s best 50.59s to finish fourth in the 400m as world champion Amantle Montsho won in 49.77s. British record holder Shara Proctor had to settle for second in the long jump in 6.65m, Russia’s Yelena Sokolova winning in 6.70m.