The problem with calssifying GID as a mental disorder is, as Jenny says, the association with 'fruitloop.
The psychiatric comminity and the medical/social community in general have, for many years, been attempting to alter public perceptions of mental illness to something as acceptable as a common cold.
Mental illnesses, depending on what you define them as, are certanly very common. But the essential problem with mental illness is the loss of liberty. This leads many people who might otherwise suffer some loss of function to abandon many others, becoming increasingly dependant.
Sadly, the medical community has been reluctant, to say the least, to abandon its authority over and especially power over people with mental health problems.
In a city here in the UK, a few years ago, during a reorganisation of medical services, the assessment and treatment of Multipul Sclorosis was transfered to a building that had formally been the admissions unit for mental health services. The mental health admision being transfered to a new building opposite.
From a medical point of view, this made good sense.
The reaction from people suffering from MS, their families, the press and the public was outrage.
The mental health services cried their usual self pity of being misunderstood by ignorant ordinary people. The reality was and is, the mental health workers so addicted to the power they weald, they are incapable of seeing that the biggest problem for people with mental health problems is the cure itself.