Gatlin won in a blistering 9.8 seconds at Hayward Field to edge out young pretender Trayvon Bromell who was second in 9.84sec with Marvin Bracy third in 9.98sec.
The win sends the 34-year-old 2004 Olympic champion to Rio as the biggest threat to reigning champion Usain Bolt.
Gatlin, twice banned for doping offences during his controversial career, had earlier signalled his readiness by setting a season-best 9.83sec in the semi-finals.
"I was trying to get me a ticket on that boat going to Rio," a delighted Gatlin said afterwards.
"My family has a ticket and that put pressure on me to punch my ticket," said Gatlin, who confirmed he will not race again before Rio after running in the 200m here this week.
Asked if he had a message for injured Jamaican rival Bolt, Gatlin smiled and gave a thumbs up. "I don't know how you print this -- but you can print that. Emoji thumb," he joked.
'Greatest ever' Felix
Gatlin's season-best time was matched in the women's 400m by Felix, who kept her dream of an Olympic 400-200 double alive with a gutsy victory hailed by her coach Bob Kersee as her best ever.
Felix, who has been hampered by an ankle injury following a freak training accident in April, surged home down the stretch to win in 49.68sec, the fastest time in the world this year.
The 30-year-old from Los Angeles is aiming to become only the third woman after Valerie Brisco-Hooks of the US in 1984 and Marie-Jose Perec of France in 1996 to have won the 200m and 400m in the same Games.
"I don't even think I can put it into words," said Felix. "Two months ago I couldn't even walk so to be here and still have everything come together I don't know it happened.
"I just know that I had an amazing team of people that didn't let me face it alone."
Felix had been in fifth coming off the final curve before finding an extra gear to pull away.
"I didn't even know where I was I just gave it everything I had," said Felix, the reigning Olympic 200m champion and 400m world champion.
Asked to comment on Kersee's comment that the win was her greatest performance, Felix smiled and commented: "Well, I trust Bobby."
Elsewhere Sunday, English Gardner won the women's 100m in 10.74sec ahead of Tianna Bartoletta in second and Tori Bowie in third.
There was another season best in the men's 400m, where LaShawn Merritt suggested he is ready to reclaim the Olympic crown he won in the 2008 Beijing Games with victory in 43.97sec.
Merritt, whose career was derailed by a string of failed dope tests in 2010 which led to a 21-month ban, has now clocked the fastest times in the world over both 400m and 200m this season.
The 30-year-old produced a devastating finish to accelerate clear of Gil Roberts and David Verburg in the final stretch.
"I worked the last 50 a little bit more than I did in the previous rounds," he said. "I still feel like I had a little bit left so I'm pleased about it."
In other events on Sunday, double Olympic decathlon champion Ashton Eaton confirmed his place in Rio by completing a straightforward victory in the multi-event discipline.
Eaton finished with 8,750 points, well clear of Jeremy Taiwo with 8,425 and Zach Ziemek in third.
In the women's high jump, teenage pretender Vashti Cunningham -- the daughter of legendary Philadelphia Eagles quarterback Randall Cunningham -- confirmed her Olympic ticket with a second place finish behind Chaunte Lowe.
"To be able to go to the Olympics, I'm more happy than disappointed about finishing second," said Cunningham, who cleared 1.97 meters, behind Lowe who jumped 2.01.
"I always want to win, but this is something that will hit me on the head and make me work harder. I've got more motivation for Rio," Cunningham added.