I don’t think you test baby gender via the position of the placenta. And I don’t think chromosome tested (in your case Down syndrome test) the position of the placenta either.
And my placenta position is placenta posterior low and I’m pregnant with boy.
I got to know my baby gender via NIPT test (harmony test - chromosomes test I. 1st trimester and further confirm by ultrasound scanning. I only got to find out about the position of my placenta in 2nd trimester detailed scanning.
Don’t be overly concerned about the gender of the baby, what is more important is baby and you are healthy. If your placenta is low, do be mindful of the risk and consult your gynae on what to avoid and what to watch out for.
Take good care and God bless you and baby!