Ravenclaw said: "Because there are just too many great roles for women of a certain ageand not enough roles for men?
And what would the text gain? It seems like an experiment that wouldn't pay off in any kind of storytelling sense. In Company, when Bobby became Bobbi, themes in the show resonated differently, as a woman's experience being told she needs to settle down and get married before she becomes too old is different from that of a man in our culture, and the text was able to explore the themes in a new way. With a domineering Stage Father who puts all his hopes and dreams onto his children because he was "born too soon and started too late"? I don't know what this major change would do for the show, other than allow Billy Porter to belt out some classic tunes, and if that's all you want, you might be better served just hearing him sing them in a solo concert."
All of these are good points. I definitely agree that "what would the text gain" is the primary question that the director would have to answer in the prep process, probably in conversation with the actor. But I think there very well might be a satisfying answer out there - as long as they do that work.
Personally, I think the most intuitive way of playing it would be to keep June and Louise as women, and play with the idea that "Papa Rose" is a closeted gay man living in an era that won't let him be himself, so he uses his daughters to enact a kind of performative femininity that he is denied. But that's just off the top of my head - again, the nuances of that lens would be worked out during prep/rehearsal.
As for the question of taking away a role from a woman, I would say that's worthy of concern, but it depends on the context. If this were to be done on Broadway, I might agree that it would be potentially unfair to take away this major, potentially career-defining gig away from a female actor in place of a man. But I think if this were done at a major LORT theatre, or for a limited run at an Off-Broadway house, not much is lost by casting a man in a single production of Gypsy. There have been many productions of Gypsy, and there will be many more productions of Gypsy, so doing a single production experimental with a man ultimately won't cause much harm. Unless it catches on to a degree that everyone starts casting the role with men, in which case I do think that will be problematic.
And as for the "she was a real person" argument - please. This is theatre! Stuff like this is done all the time, and it really doesn't matter. Unless it perpetuates a massive misconception (e.g everyone starts thinking the real Rose Havok was actually a man), but that's highly unlikely.