The entire reason for GDPR is to prevent unscrupulous data sharing by companies and other entities who strive to make money out of your private data. The crux of GDPR is that if you ask customers for personal private data, you may not share it, willy nillie without the customers full consent to do so... full stop.
In my one business, collecting data, requires that the banks and providers of credit MUST obtain the consent of their customers, to pass the information they acquire onto a credit bureau. If they pass on information of an adverse nature, without the permissions of their customers, then not only are they in breach of GDPR and similar national regulations, they may also find themselves in the dock for libel (or as we term it defamation).
WikiTree is not making money out of publishing data volunteered by its membership. If it were, then it would be seriously in breach of GDPR as it pertains to EU citizens. I have opined a different view, for point of discussion, and that is whether publication, which is what WikiTree does, of the data its members have volunteered (i.e. consented to) would in anyway be in breach. Clearly it is not, because Wikitree still functions.
Now to the question of a public person. I can see no reference in GDPR concerning the publication of information generally in the public domain about a living person. This is an entirely different concept and I argue that it is outside the domain of GDPR. That is why, in the case of Robert Mugabe, you can find much reference to him in newspapers, books and on the internet. Whether its a book or an internet site, it is still data! Wiki tree would not be sending data streams to a third party for purposes of marketing or some other nefarious activity.
The only concern we would have in publishing Robert Mugabe's profile would be that it is in no way defamatory. WikiTree would need to have a due diligence process and before that we would need to clearly define what constitutes a public person. I just simply cannot see that there would be any breach by WikiTree of the GDPR regulations... Robert is a Zimbabwean citizen, not an EU citizen.