A common refrain among trans activists is that the World Health Organisation has removed gender dysphoria from the ICD (International Classification of Diseases).
It is true that in the final draft of ICD-11 (due for final approval next year), gender dysphoria (previous known to the WHO as gender identity disorder, and known in ICD-11 as "gender incongruence") is moved out of the "disorders" section. The main rationale is to reduce stigma.
For that matter, the UK government had already stated as long ago as 2001 that it didn't regard gender dysphoria as a mental illness. So for trans activists to use its status as a non-illness as a reason for getting rid of the medical evidence required for a GRC is a non-sequitur, since the requirement for medical evidence doesn't rest on gender dysphoria being categorised as a disorder.
The fact is that although it's no longer categorised as a disorder, it still appears in ICD-11 and still has a list of associated diagnostic criteria and information on associated treatments. So it makes perfect sense that medical evidence should still be required for a GRC.