As my name suggests, I do not agree with @Tandora on most things about gender identity. But they are spot on about this.
Previously "gender identity" was seen as a socially formed idea/awareness of oneself as a girl or boy. Because humans develop in a social context. (In the same way one might develop, say, a national identity.) Children start off picking up on broad stereotypes ("I'm a girl, so pink is my colour", "boys have short hair") and as they develop and their thinking becomes more complex, it becomes more nuanced and they can see that there are many examples that do not fit rigid stereotypes, and understand that biological sex is the one constant difference between men and women.
Obviously this development is based on knowing they are male or female; we tell them they are a boy or girl as observed at birth. But the internal feelings are based on social experience.
Trans supporters appear to believe that this gender identity is in fact innate.
Or possibly, they don't believe it is, but would prefer a world where biological sex wasn't the determining factor, where young children could choose at an early enough age so that they have the relevant social experiences to develop a gender identity associated with the opposite sex. This is why social transition is not a neutral act.
Why anyone would want to do that, I don't know. You either end up with an a young person who has to painfully come to terms with being the opposite sex to how they have perceived themselves, or one who's identity is so entrenched in believing they are the opposite sex that they commit to a (shortened) lifetime of health-ruining medication and (poor, experimental) surgery on healthy body parts. Neither are good options.