I've seen some people attempt to make a voice gender recognizer through machine learning, but nothing that has actually worked well. I'm not an ML expert, but I expect they're overfitting on just a small subset of accents and specific formants. The one I played with (can't find the link) was just trying to show weight vs resonance, and it got very confused by my voice. I am never misgendered from my voice but I (intentionally) have a slightly deeper femme voice than what it had been trained on.

IMO, the best way to learn to control your voice is to learn to hear variations in size, weight, pitch, open quotient, prosody, etc. From there you can become your own coach so you're not focused on an app while having a real world conversation with someone. Would a gamified version help? Possibly.

(sorry for the delay, I have no idea if you'll see this. I wrote it last week but apparently never pressed the submit button??)