Gatlin, 37, suddenly winced in pain around the 80m mark of the 100 metre sprint at the Iaaf meeting in the Croatian capital.
The American slowed down and finished fourth with a time of 10.29 seconds. His compatriot Michael Rodgers won the race in 10.04 seconds.
On Wednesday, Gatlin's agent, Renaldo Nehemiah, told Reuters that tests carried out on the 2004 Olympic gold medallist had not revealed any serious injury.
He added Gatlin would be back in training within four days.
Gatlin won gold in London in 2017 at the world championships ahead of his fellow American Christian Coleman and Usain Bolt from Jamaica