The 30-year-old brought a wealth of experience to her first official race ride, aboard Quaintrelle.
She produced a polished and well-timed ride over the 1600m distance to leave Wingatui with a perfect record of one ride for one win.
Quaintrelle was buried along the fence for much of the race, then Gestain weaved a path that a senior jockey would have been proud of to get her mount into the clear to win.
Gestain headed an all-women rider trifecta in the race.
Local Emma Anderton, who was also having her first ride in an official race, finished second with Manhatten Fire.
Nicola Brown, a previous winner of the annual amateurs' race, was third on Rich Habit.
The quietly spoken Gestain said she got a massive thrill from her victory.
The jockey refined her polished riding style after working in stables across Europe and Australasia.
A two-year stint with the powerful Godolphin stable and an extended stay in Queensland are among her experiences.
Gestain is on her second stint with Yaldhurst trainers Michael and Matthew Pitman.
Michael Pitman, who has returned to New Zealand after campaigning Enzo's Lad in the United Kingdom and meeting the Queen, was on hand to watch Gestain's win.
He said in an acceptance speech that Quaintrelle's victory was one of the most satisfying wins of his career.
"I am not going to go into it too much, but this is one of the most satisfying wins of my career," he said.
"We have had to fight and fight officialdom for Meghann to have a licence.
"You can see how well she rides. It is an absolute joke that we have had to go through what we have been through."
The Pitman stable scored a winning double when Meara Mary won in maiden company over 1200m for rider Tanya Jonker.
