Gemechu, still just 21, took 50 seconds off the time she ran in 2018 to win the $27,000 first prize at the IAAF Gold Label road race and also pocketed a course record bonus of $10,000.
The women's race came down to a thrilling head-to-head duel over the final five kilometres between the defending champion and her compatriot Yelamzerf Yehualaw.
A large pack went through 5km in 15:43 before a sharp increase in pace by the male pacemakers brought the leading group of six past 10km in 31:22, indicating that the course record was very much under threat.
One-by-one runners became detached until only Gemechu and Yehualaw remained as they went through 15km in 47:05.
Coming into the final kilometre, Yehualaw - who won the African Games half marathon title in August - edged in front and briefly looked like she was going to cause an upset but with the finish line approaching Gemechu dug deep and managed to claw her way past her rival to also notch up a personal best while becoming just the second woman to defend in Delhi.
"I was tired after Doha (where she finished fourth in the world championships 5000m two weeks ago) but I wanted to come here and fight for the course record. I knew I was mentally strong," reflected Gemechu.
Yehualaw, even younger than the winner at 20 years, took more than three minutes off her previous best for the distance when finishing just one second behind Gemechu in 66:01.
Zeineba Yimer, who started to lose contact with the leading pair just before 15km, held on to make it an Ethiopian 1-2-3 when she crossed the line in third place in 66:57, the same position as she had finished in 2018.
Kenya's 2017 world cross country champion Irene Cheptai may not have made the podium but had a satisfactory half marathon debut when she came home fourth in 67:39.
( With inputs from IANS )