I would say it's not reductive at all. Why does she hate the Vistani? Because her Vistani mother cursed her to never be able to have "a man, a babe, a home" (as per Black Box) because such things would only bring her tragedy.
Villains are supposed to be interesting; they are often much more interesting than the heroes. A villain whose entire identity is "likes to screw others over because she can't have a man" is a high school villain.
You are making a lot of interesting points but I am finding I only have time today to hit on one or two (since some of this requires me sifting through the old Ravenloft books). Right now I am tackling the threads I can. Will try to return to these later
Her Vistani mother didn't curse her. Her Vistani mother warned her, because she was the daughter of a very evil man (someone whose identity is only implied in the Black Box). It is one of the single most intriguing entries in the book for that reason. And what burns Gabrielle isn't that she can't have a man. You are simplifying her backstory to fit a narrative that does not match the character. She wants the things that in her mind make for a normal life: a home, a child, a husband; and she wants to be accepted as a vistani by the vistani. She is an outcast among outcasts (because the Vistani reject her for not being pure Vistani, even though her bloodline goes back to Madame Eva); and she burns with resentment for people who can live a normal life. This, again, is a highly relatable story and not one unique to a woman who 'wants a man'. Anyone who has longed for something in life that was 'normal' that they couldn't have, because of accidents of who they are or what befell them, will find something that resonates her. The mystery around her father alone was wonderful (and I loved that it was hidden there if you knew where to look in the black box, but they never actually came out and said it).
Further what makes an interesting villain is a good backstory (which I would argue she has in spades), popping off the page, being both appealing and repugnant at the same time; and being terrifying. She meets all those qualifications handily. Obviously this is subjective. All I can say is I thought this was a great character from the very beginning, one of the stand out characters in fact (maybe because of family background a domain lord and domain built around the evil eye had special appeal to me); she is obviously a common gothic trope too (she is described as a witch in the domain entry, which is how the inhabitants see her). But this was definitely one of the domain lords I instantly knew I could use, and a domain I immediately could imagine and think of great adventures to set in.
I would argue very strongly she isn't a high school villain. This is a common trope, taken to a whole new, very memorable level. In fact one of the worst pieces of meta plot, IMO, was having her son become the political leader of Invidia. That character was a pale imitation of her, and a product of meta plot (which is something I think the line deserves criticism for in the 90s). She was a vibrant villain. He was terrible and boring. Having Gabrielle Adere couple with the Gentleman Caller to produce Malocchio, now that is something I can get behind criticizing. It felt very out of character, forced, and it really hurt her as a villain in my opinion (the Gentleman Caller really was one of the worst entries in the setting in my view).
It's lazy because the male characters get intriguing backstories and goals while the female ones don't; because their characterization is centered entirely around the ideas that a Complete Woman needs a man and a child in order to be happy, and a Woman who doesn't have them is Incomplete and Evil.
This isn't lazy. It isn't interesting or uninteresting on its own. And I think you are projecting messages on to the backstories that are not there. A lot of these are just characters who want love. Maybe there are more male characters motivated by that (as there are more male dark lords and I agree there should be more female dark lords). But maybe the problem is not enough male characters are motivated by that kind of romantic desire, or by a desire to have a normal life. I tend to doubt it, as I think a lot of the male dark lords were motivated by similar things. But I would honestly need more time to assess. I am not saying you are correct either, that it was present and ubiquitous in old Ravenloft (I think you are actually wrong, but to tackle that topic with the length it deserves would require an amount of review that I just don't have time for today).
All this is, I think, is something that doesn't line up with current views on gender. Which is fine. You can talk about that if you want. But I don't think that is a matter of writing or design quality on its own. That is more a matter of whether the content is ethical (which we could debate all day). Again, this line is 30 years old. I think not only are people not understanding it in the context of its time (where many of these tropes would have read very, very differently: for example in an age when Riot Grrrl was a big thing, the idea that female characters were motivated by a hatred of men would have been transgressive and in some instances seen as empowering (not saying they were all motivated by that, or that they should be, but I think the 'message' is very different 30 years ago in that context than it is today, and a lot of the discussion here seems to be losing that context: that is one reason why I keep reminding people there were so many female writers involved in Ravenloft). But just having a checklist of things that match modern sensibilities, and making sure domain lords meet those things: that doesn't make it more interesting in my opinion. And that, is definitely closer to lazy because you are literally just going down a list rather than sitting there and thinking about it, making it interesting and compelling. It just makes it more in line with current sensibilities (or perhaps not, I am not sure all the critiques here reflect current sensibilities).