The medical condition, gender identity disorder, exists.

Except officially it doesn't - not anymore. GID - gender identity disorder - used to be considered a psychiatric disorder.

But the American Psychiatric Association retired that term in 2012, changing it to "gender dysphoria" in the DSM-V. More recently, the APA has been making changes to the definition and criteria for "gender dysphoria" in its written materials to further keep up with the demands of activists, and to widen it so that more people can receive a clinical diagnosis.***

WHO scrapped the term GID years ago, calling it "gender incongruence" instead. Then WHO started using the term "gender dysphoria." In 2019, WHO removed all conditions related to "gender" from the International Classification of Diseases (ICD), which sets the global standard. https://www.cbsnews.com/news/world-health-organization-removes-gender-dysphoria-from-list-of-mental-illnesses/

People who have this condition, and treat it by living as the opposite sex, are transgender.

This is out-of-date too. Now the APA says "gender dysphoria" in adults, adolescents and children isn't necessarily about wanting to be or live as the opposite sex, believing that you "feel like" the opposite sex, or insisting that you are the opposite sex. Rather, gender dysphoria today can also mean wanting to become, desiring to be treated as, or believing you have the feelings typical of "some alternative gender different from one's assigned gender." https://www.psychiatry.org/patients-families/gender-dysphoria/what-is-gender-dysphoria

Finally, many transgender people and advocates say it is not necessary to have any kind of "gender dysphoria" or distress to be transgender.

Not all transgender people suffer from gender dysphoria and that distinction is important to keep in mind.

https://www.psychiatry.org/patients-families/gender-dysphoria/expert-q-and-a

https://everydayfeminism.com/2015/08/not-all-trans-folks-dysphoria/

as awareness about the trans community has grown, so too has the number of people who identify as transgender. Yet, as the community grows, more and more people who identity with the label of transgender have also found they haven’t ever felt any dysphoria at all. Instead they learned they were trans for a variety of political to social to emotional reasons.

we cannot let dysphoria be the only path, the price of entry, into our community. It frames being transgender as something painful, shameful, and to be resented. It defines transgender not as something to be proud of, but to be fought against, or hide.

And this view fosters not only internalized transphobia, but transphobia from others. Go into any comment section on any transgender content, and I’ll place bets you’ll find someone decry that transgender people are mentally ill somewhere in there. If indeed gender dysphoria, a mental illness, is a requirement for being transgender, would that mean they are that far off? And, as I already stated, if you treat gender dysphoria, have you cured your transness too? Is being transgender something we really want to be curable?

https://www.advocate.com/commentary/2019/1/18/do-you-need-gender-dysphoria-be-trans

According to the current thinking, your views make you a "transmedicalist" or "truscum" (sorry if the latter sounds offensive; it's not my term, it's the term used by those who object to the idea that dysphoria is necessary to be trans).

*** The APA is constantly expanding and loosening the diagnostic criteria because the more people who get a diagnosis, the more powerful the gender lobby will become and the more individuals - including kids and adolescents - will be fed into the maw of Big Gender Medicine and become lifelong patients. With Biden's recent rule change, now in the US everyone wanting medical interventions for "gender affirmation" will be able to obtain insurance coverage for their hormones & surgeries from private providers and publicly-funded programs like Medicaid and Medicare alike. Ka-ching! Ka-ching! Ka-ching!