Right, so, this is at the heart of the debate - and this has to be VERY clear.
We need to be absolutely clear on what mental illness means. You're not mentally ill if you're not in mental distress.
So what you wrote below is correct but then you got it wrong on this comment: gender identity disorder is a mental condition; it is mental distress caused by the gender identity they've been given. When they can adopt the identity that is more true to themselves, the distress goes away.
Transgenderism is not the illness: it's the treatment for GID. Kids stop killing themselves when they're true identity is embraced and supported - internally and externally. And that's supported by 240 years of medical research that critics of transgenderism never read.
Mental illness is not a bucket of social behavior we don't like - it was in the 1850's, but we've abandoned that definition. We are much more clear today about what that is, what it means, and what action we need to take.
That is why I argue back with you - because your understanding of mental illness is not medically up to date.