Bell's report wasn't leaked at that point, I don't think.
The Observer article was mainly about the parents who wrote to the Tavi to express concern about the fast-tracking of a medical pathway for their children. The letter to the Tavi by parents had been seen by The Observer.
The article then refers to the Bell Report (as we now know it) but states that "The Observer believes that the report questions whether the clinic should do more to consider young people’s personal histories, notably by examining whether they are on the autistic spectrum, have experienced trauma or are being influenced by social pressures, before helping them on the path to transition".
So it looks like The Observer didn't actually see a copy of the report at that point. But clearly someone at the Tavi had been speaking to The Observer back in October/November last year.
I wasn't aware of this though, about the Tavi's initial response to the Bell Report.
“The trust is concerned by the tone and manner in which these allegations have been made. They reveal a negative attitude to gender dysphoria and gender identity which does not reflect the views or the approach of the trust.”
So the Tavi was concerned about "tone" and "manner" of the Bell Report and now Gilligan is reporting on the actual content of the report, they are "very disappointed at the unauthorised disclosure of Dr Bell’s confidential report to the media.”
They are not contesting the truth of the report, but don't like the language it uses to deliver it apparently, nor the medium in which it is being discussed.
Cos that's the important thing; it is not like young lives are being ruined.