The 33-year-old American, who has served two doping bans, recorded a time of 19.57 seconds in Oregon.
It bettered the 19.68 secs he ran in May which, at that stage, was the best in the world.
"I wanted to go out and make a statement, and that's what I did," Gatlin said.
Gatlin has been the dominant sprinter in 2015, having also achieved the quickest time in the 100m (9.74 secs).
He will go up against 100m and 200m world-record holder Usain Bolt in both events at August's World Championships in Beijing.
In a race run in a legal wind of 0.4 metres per second, Gatlin finished ahead of Isiah Young (19.93 secs) and Wallace Spearmon (20.10 secs).
