Gebreslase broke away with about two kilometres to go and never gave Korir a chance down the final stretch, smashing Briton Paula Radcliffe's previous mark of 2:20:57 set in 2005.
Israeli Lonah Salpeter (33) seized a chance for the third spot on the podium after Ethiopian Ababel Yeshaneh (30) fell off the lead pack with less than 10 kilometres to go.
Kenyan Ruth Chepngetich (27), who won three years ago, dropped out at around the 18-kilometre mark due to stomach issues, as the runners battled under sunny but pristine conditions, with temperatures hovering above 10C.