Here’s the Wiki page: en.m.wikipedia.org/wiki/Gender_... And a quote from the Endocrine Society (the oldest, largest and most active such organisation). “Considerable scientific evidence has emerged demonstrating a durable biological element underlying gender identity.”
Many countries have also outlawed conversion therapy aimed at changing gender identity. Its standing as a widely accepted idea is without question. Your disbelief is the anomaly. I’d suggest not just relying on your nan for scientific information if they’ve misled you in this (fool you twice etc).