It's a bullshit claim.
Sadly, mental health provision for all teens, regardless of diagnosis, is a shambles, unless you can pay for it privately.
So, regardless of diagnosis, if you are also experiencing suicidal ideation and you have no or inadequate mental health support, you are at risk of acting on that ideation.
There is no evidence that this risk is high in children with a GD diagnosis as compared to other diagnoses, and the risk remains relatively low in children anyway.
Anyone other than TRA's would think this a cause for celebration.
I have read a study that suggested gender non-conforming and bisexual girls had particularly poor mental health, but I'd have to go and dig it out.
My suggestion, for anyone for whom this inappropriately inflated fear is a live issue, is to find a GC therapist for their child - either DBT or work with a Jungian therapist are the most helpful, but DBT for children and teens is almost impossible to access.