After Bolt edged the American by .01 second in Sunday's 100 final, he is now running in his favored race and seeking his record 10th world title overall.
Bolt was grinning widely after crossing in 19.95 seconds, coasting for about 60 meters and checking the stadium screens high up in the stands of the Bird's Nest to see if someone could come close. As usual, none did.
In the previous semifinal heat, Gatlin, unbeaten over two years in the 200, ran 19.87 seconds but worked a little longer than Bolt before easing up.
On a night when javelin thrower Julius Yego extended the surprising lead of the Kenyans in the medals table to five gold, two of his teammates were provisionally suspended following initial positive doping tests.
The IAAF said it targeted Joyce Zakary, a 400-meter runner, and hurdler Koki Manunga for testing after competing in Beijing. Zakary ran a national record in the women's 400 but the 29-year-old Kenyan did not start in the semifinals. Manunga failed to reach the semifinals in the women's 400 hurdles.
On the field, Yego set the best javelin mark in 14 years with a throw of 92.72 meters, good enough for an African record on top of gold.