It is a post that was directed to a private group - that group in this case is Facebook account holders. So by taking that information and posting it here, it may run afoul of the GDPR. I have seen arguments that even forwarding a group email to someone not on the original recipient list is a violation, so this wouldn’t be any different. It’s a communication that is behind an authentication wall for a reason.

Those arguments are wrong.

The GDPR deals with processing[0] of personal data[1], not data about anything else any entity wants to remain somehow restricted. Other legal regimes may deal with disclosures of that kind, but not the GDPR.

[0] ‘processing’ means any operation or set of operations which is performed on personal data or on sets of personal data, whether or not by automated means, such as collection, recording, organisation, structuring, storage, adaptation or alteration, retrieval, consultation, use, disclosure by transmission, dissemination or otherwise making available, alignment or combination, restriction, erasure or destruction;

[1] ‘personal data’ means any information relating to an identified or identifiable natural person (‘data subject’); an identifiable natural person is one who can be identified, directly or indirectly, in particular by reference to an identifier such as a name, an identification number, location data, an online identifier or to one or more factors specific to the physical, physiological, genetic, mental, economic, cultural or social identity of that natural person;