I don't think they are fully interchangeable, I would not presume to know someone's gender even if I knew their sex, or vice versa. In aquaria people usually ask for breeding reasons - in which case I think gender is the wrong term, but it appears an increasingly common usage - language changes.

We use them interchangeably. You don't ever need to know someone's assigned sex unless you're their doctor so it doesn't matter, we are always talking about gender. Using a country where trans women are killed to claim oppression happens on the basis of sex makes you an obvious bigot.