Gemili produced the fastest time from a British athlete in the event since 2005 as he won his heat in a time of 20.17 for the joint-fastest time of the round along with South Africa's Anaso Jobodwana.
The time put him in fifth on the UK’s all-time list and second in the U23 category as the 19-year-old from London made an impressive start to the event, along with compatriot James Ellington, who also won his heat.
"Maybe I didn't expect it this early but coming into the 200m I knew I was in shape and I just wanted to execute," said Gemili, who was frustrated not to make the 100m team.
"I looked across and thought someone else must have got it but there's more to come and I just want to hit it this afternoon."
Two-time defending champion Usain Bolt jogged through his heat to win it in a comfortable time of 20.66.
Bolt, who took gold in the 100m on day two of the championships, pushed Britain’s Delanno Williams into second.
The 26-year-old Jamaican, who won the 200m titles in Berlin in 2009 and Daegu in 2011 and is also double Olympic champion and world record holder in the event, clocked an easy-going time as he cruised away from the field.
"I'm not really a morning person so I'm just happy to get through," Bolt said after advancing as the 21st-fastest qualifier.
"My foot is feeling better. It was sore but we've been working on it for four days."
Bolt, who set the current world record of 19.19 in Berlin four years ago, is the fastest in the field this season with 19.73 in Paris six weeks ago.
He will be joined by all the main favourites in Saturday's semi-finals, with the final to round off the evening session of day eight of action at the Luzhniki Stadium.
Jamaica team-mates Warren Weir, the Olympic bronze medallist from London, Nickel Ashmeade and debutant Jason Livermore all went through.