Wait... How old is sherry? and how old is Gin? >__<
I don't think they had any romantic relations. If they had, I doubt Gin would continue to refer to her as Sherry. It would probably be something like Shiho, that woman, or just that bitch or something.
Unless she is a person that Gin looks up to, or maybe Gin's eternal rival or something like that. But really... I don't think they have any special relationship with each other or at least not directly.