What you're referring to is biological sex. Under current medical technology, chromosomes are immutable, and genitalia are excruciatingly difficult to change. If your body's biological base levels are male, that's not gonna change, and if your body's biological base levels are female, that won't either. The vast majority of trans people acknowledge that.
Gender, on the other hand, is a more fluid concept. It's how one presents themself from a social and societal standpoint, in order to reflect how they feel internally. And there are many factors that play into this, including genetics, neurology, and hormones. It's been medically proven that this is not just something that people can turn on or off, and that goes overlooked a lot.