RWarehall: Both. It's private information between GoG and that user. Revealing it was wrong without permission. Just as GoG shouldn't discuss their interactions with developers. That you were told anything about it is unprofessional.
We are talking about a nickname, not real name or any real personal information, only nicknames.
Also you talk about "
business etiquette" but, as somebody who signed lots of NDA during my career, I can tell you that what you call "unprofessional" or "against business etiquette" is kind of the purpose of NDAs and is pretty common.
Let's say for example that Gog has a NDA with company Y for the maintenance of their servers / database (could be external consultants, Microsoft, etc...), then it could mean that this company could potentially (depending of the term of the NDA) have access to all the data in Gog servers, including very sensitive and very private users information like real name, billing information, etc... (Again it's an example I have no idea how Gog manage their IT)
It's something pretty common, at lot of big companies no longer handle their data, support, or computer parks themselves but rely on external partners to do so and we are talking about companies that have very sensitives information about their customers.
So in comparison sharing a couple of nicknames with peoples whit witch they have signed a NDA is incredibly benign.