The Cats gave up a 28-point third-quarter lead to go down by four to Melbourne 8.8 (56) to 8.4 (52) at IKON Park on Saturday.
Chloe Scheer had a chance to win it late for Geelong with a set shot and only seconds remaining, but her attempt drifted wide, sending the Cats to the semifinal round and the Demons to a preliminary final.
Moama’s Gregor started at fullback alongside Chantel Emonson, and helped Geelong’s defence keep a handle on Melbourne in the first half, allowing only 10 points.
Gregor had seven disposals, eight tackles and six intercept possessions.
Also in action for the Cats was Kyabram’s Julia Crockett-Grills, who came off the bench and logged 13 disposals and three intercept possessions.
Geelong will play Sydney, who ousted the Western Bulldogs in an elimination final, at their home, GMHBA stadium, this Saturday from 4.05pm.
The winner will face the daunting task of taking on two-time premier North Melbourne in the prelim.
Melbourne will take on the winner of Carlton and Hawthorn in the other preliminary final.