American Gay, who beat Jamaican world record holder Bolt in Stockholm last week, defied the cold, damp evening conditions and, after a flying start, finished clear of the field. Jamaica's Yohan Blake finished second in 9.89.
"I wasn't expecting to go that fast in these conditions but I knew I was in good shape and was going to go fast. I was hobbling a bit at the end — my groin is a little sore but I'm sure it's OK," said Gay.
Gay also joined the ranks of the walking wounded and was limping after his impressive display, which was a fitting finale to the first day of the meeting that concludes on Saturday. American Bernard Lagat spoiled double European champion Mo Farah's homecoming in the 3,000 metres by pulling away from the Briton and Australia's Collis Birmingham on the final bend to clinch victory in seven minutes 40.36 seconds.
Farah, who won the European 5,000 and 10,000M titles last month, was roared on by a capacity crowd desperate to see him win but for once he had to settle for second in 7:40.75.
Sweden's former Olympic, world and European title holder Christian Olsson showed he was heading back to form after struggling with injuries for the past four seasons, winning the event with 17.41 ahead of France's Teddy Tamgho.
Results: Men: 100M: Tyson Gay (US) 9.78 seconds, 1; Yohan Blake (Jam) 9.89, 2; Richard Thompson (T &T) 10.05, 3. 800M: Abubaker Kaki (Sud) 1:44.38, 1; Andrew Wheating (US) 1:44.56, 2; Nick Symmonds (US) 1:45.28, 3. 400M hurdles: Bershawn Jackson (US) 48.12, 1; Javier Culson (Pue) 48.17, 2; David Greene (Bri) 49.09, 3. 3000M steeplechase: Paul Kipsiele Koech (Ken) 8:17.70, 1; Ezekiel Kemboi (Ken) 8:19.95, 2; Brimin Kipruto (Ken) 8:20.77, 3. Pole vault: Lukasz Michalski (Pol) 5.71 metres, 1; Derek Miles (US) 5.61, 2; Przemyslaw Czerwinski (Pol) 5.51, 3. Triple jump: Christian Olsson (Swe) 17.41, 1; Teddy Tamgho (Fra) 17.27, 2; Alexis Copello (Cub) 17.02, 3. Women: 200M: Allyson Felix (US) 22.37 seconds, 1; Debbie Ferguson-McKenzie (Bah) 22.88, 2; Sherone Simpson (Jam) 23.04, 3. 1500M: Nancy Jebet Langat (Ken) 4:07.60, 1; Anna Alminova (Rus) 4:08.82, 2; Lisa Dobriskey (Bri) 4:09.07, 3. 5000M: Tirunesh Dibaba (Eth) 14:36.41, 1; Vivian Cheruiyot (Ken) 14:38.17, 2; Sentayehu Ejigu (Eth) 14:39.24, 3.100M hurdles: Priscilla Lopes-Schliep (Can) 12.52, 1; Sally Pearson (Aus) 12.61, 2; Lolo Jones (US) 12.66, 3. High jump: Blanka Vlasic (Cro) 2.01 metres, 1; Ruth Beitia (Esp) 1.91, 2; Irina Gordeeva (Rus) 1.91, 3. Long jump: Darya Klishina (Rus) 6.65, 1; Lyudmila Kolchanova (Rus) 6.65, 2; Hyleas Fountain (US) 6.57, 3.
Shot put: Nadezhda Ostapchuk (Bel) 20.27, 1; Valerie Vili (NZ) 19.83, 2; Gong Lijiao (Chn) 19.26, 3. Discus throw: Yarelis Barrios (Cub) 65.62, 1; Sandra Perkovic (Cro) 63.30, 2; Nicoleta Grasu (Rom) 61.78, 3.
Javelin throw: Barbora Spotakova (Cze) 63.50, 1; Kara Patterson (US) 63.41, 2; Linda Stahl (Ger) 59.60, 3.