[user104658]

I think if you strip it right back, at its most simple level actual transgenderism is a form of body dysmorphia.

The gender stuff that doesn't include a desire to physically transition is something different and I would say largely social. Not a "mental illness" in any more meaningful a way than, for example, any religion and frankly there'd be far fewer problems if it was simply considered a belief system in the same way as a religion.

There is of course always the risk that the social aspects can develop into full dysmorphia especially if the individual already has underlying mental health issues like depression, anxiety, trauma or general identity issues.