I rewatched the epilogue and noticed something quite interesting. When Fio narrates that Gina has become more beautiful every year, a panning shot of the Hotel Adriano's outdoor restaurant is shown. Toward the very back is an elegant woman dressed in blue and a mysterious man in black walking together into the hotel.
I think this is Gina and her lover (he's about Curtis's height; Marco's around Gina's height). The bet was that the man she loves would visit her in the garden during the day and they will fall in love. This would explain the empty gazebo in the final scene (she's found love and no longer waiting). As for the red plane parked outside Gina's garden, I think it's meant to show that Marco's come full circle with his past and sense of unworthiness, rather than a romantic resolution.
So did Gina win the bet? Yes. Perhaps just not in the way we were led to believe. And this is why Fio keeps it their secret—to give Gina's story dignity and closure. This also explains why Gina and Fio could become and remain close friends through the years—their hearts no longer centered around the same man (Marco).