Bolt, who beat Gatlin to retain his 100 metres title at the Bird's Nest on Sunday, shot out of the blocks and led for the entire race before easing up over the final 15 metres to cross the line in 20.28 seconds.
The 29-year-old Jamaican said he was still feeling the effects of the 100 metres but will be looking to raise the pace in Wednesday's semi-finals before peaking for Thursday's final, where he could win a fourth successive 200 metres gold.
"Tired, tired, tired," the Olympic champion and world record holder told reporters.
"This means a lot more to me. I'm a little worried about my fitness and I need to work a lot over 200m. But I'm a better technical runner over the 200m so we'll see what happens.
"I'm tired and my legs are still sore, but I'm going to have another bath tonight and, hopefully, tomorrow I'll be there."
Gatlin, unbeaten in the 200 metres since 2013 and the 2005 world champion at the distance before his second doping ban, was even more dominant in the following heat, maintaining his form through the line to win in 20.19.
The 33-year-old American, who owns the fastest time of the year (19.57), said he would have eased up but for the presence behind him of Japan's Abdul Hakim Sani Brown, who finished second in 20.35.
"The 100 metres final was a difficult one for me, also emotionally," said Gatlin.
"But now I'm going for 200m. I have two days to go. My race would have been slower but Sani pushed me so much. This young guy from Japan is phenomenal."
Anguillan young gun Zharnel Hughes, who trains with Bolt in Jamaica but runs for Britain, edged the fifth heat in 20.13 courtesy of his dip for the line, while Panama's Alonso Edwards ran 20.11 to win the second.
The fastest time of the heats, whoever, came from Ramil Guliyev, who ran quick times as a junior but lost a big chunk of his career to a ban after deciding to represent Turkey rather than his native Azerbaijan.
The tattooed 25-year-old ran a national record of 20.01 but said he thought he go faster.
"It was a good run, now I'll prepare for the next one," he said.
(Editing by Ed Osmond)
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