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The reigning European champion held off Danny Talbot and Zharnel Hughes on a track where he ruptured his hamstring a year ago.
British record holder Dina Asher-Smith edged out Desiree Henry and Jodie Williams in the women's event.
Matthew Hudson-Smith won the 400m in a European leading time of 44.88 seconds.
On his home track, Hudson-Smith pulled away from Rabah Yousif and Martyn Rooney and held his form down the home straight as Yousif came through in second with a characteristic late surge.
In the absence of established stars such as Mo Farah, Greg Rutherford and Jessica Ennis-Hill, Andy Pozzi produced one of the other performances of the weekend to win the 110m hurdles with a personal best of 13.31 secs into a headwind of 1.3 metres.
Pozzi, taking part in his first British championships since 2012 after four years of injury, sealed automatic Olympic selection, along with Lawrence Clarke, who was second.
Gemili had never before won a national title but he powered past Hughes in the lane outside him on the bend and then kept his form as Talbot closed in the last 20 metres to win in 20.44 secs.
A seemingly-unfit Hughes could only come fourth in 20.84, meaning he faces a shoot-out with Nethaneel Mitchell-Blake for the third and final Olympic 200m place.
Gemili, who missed the British Championships last year through injury, said: "This time last year, I was watching on TV as my hamstring was in two pieces.
"That third spot will be tough. Rather them than me. I am glad I booked my place, I wanted to do that automatically. If I didn't then it would have been a tough selection for everyone.
"I know those guys aren't 100%, but when they are they can fly."
Asher-Smith was given a scare by Henry, looking to put the disappointment of her 100m display on Saturday behind her, her margin of victory - 23.11 secs to 23.13 secs, with Williams a further nine-hundredths of a second back - testament to the depth of talent Britain has in the women's sprints.
Athletes needed to finish in the top two in their event this weekend and have two performances inside the qualification standard to be guaranteed a place in the squad for Rio.
While that was straightforward for some - Eilidh Doyle dominating her 400m hurdles final in 54.93 secs, Charlie Grice and Chris O'Hare taking an expected one-two in the 1500m - it left others with anxious weeks ahead.
The final deadline for qualification times is 11 July and Sebastian Rodger will be one hopeful chasing another quick time after winning the men's 400m hurdles but with only one mark under his belt.
But Jack Green, who has battled depression and came off his medication in order to get in the optimum physical shape, will be on the plane for Rio after coming in second with the requisite times already under his belt.
There was emotion too for Steph Twell after she won her third 5,000m title to qualify for her second Olympics, having missed out on London after an awful ankle injury, and for Eilish McColgan, who will join her after fighting through for second.
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Laura Muir and Laura Weightman both qualified from the 1500m, the former with an impressive front run, while Jazmine Sawyer produced a brilliant personal best of 6.75m in the penultimate round of the long jump to steal both the title and a debut Olympics spot.
Yet there was disappointment for Goldie Sayers, 11 times the British javelin champion, as she was beaten into second place by a personal best of 57.44m from Jo Blair. She will now need to find a throw of 62 metres or more before that July deadline if she is to go to her fourth Olympics.
Lynsey Sharp has the consolation of an Olympic place to make up for her 800m defeat by 2015 world finalist Shelayna Oskan-Clarke, but seven-time champion Michael Rimmer suffered a huge upset when beaten by Elliot Giles and Jamie Webb, and will now have to hope for the selectors' largesse.
Men:
100m: James Dasaolu, James Ellington
200m: Adam Gemili, Danny Talbot
400m: Matthew Hudson-Smith, Rabah Yousif
1500m: Charlie Grice, Chris O'Hare
5,000m: Tom Farrell, Andy Butchart
110m hurdles: Andrew Pozzi, Lawrence Clarke
400m hurdles: Jack Green
High jump: Chris Baker, Robbie Grabarz
Pole vault: Luke Cutts
Women:
100m: Asha Philip, Daryll Nieta
200m: Dina Asher-Smith, Desiree Henry
400m: Emily Diamond, Seren Bundy-Davies
800m: Shelayna Oskan-Clarke, Lynsey Sharp
1500m: Laura Muir, Laura Weightman
5,000m: Steph Twell, Eilish McColgan
100m hurdles: Tiffany Porter: Cindy Ofili
400m hurdles: Eilidh Doyle
3,000m steeplechase: Lennie Waite
Long jump: Shara Proctor, Jazmin Sawyers
High jump: Morgan Lake
Pole vault: Holly Bradshaw
Hammer: Sophie Hitchon
Discus: Jade Lally
*The full Great Britain team for Rio will be announced on 13 July
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Adam Gemili produced a champion's display under immense pressure to take the British 200m title and win his place in the GB Olympic squad.