By proof I mean a statement from at least one of the antivirus companies that developed the software that marked said executable as a virus.
??? Then why are you posting here?
To raise awareness about the issue so that responsible people that care about their data and about other people on the internet, can exercise proper caution when installing software from GOG.
Yes, indeed. I see no proof in the link you provided, only a reassuring statement from GOG. By proof I mean a statement from at least one of the antivirus companies that developed the software that marked said executable as a virus.
Sorry, but what you said just means that you trust Avast more than GOG, nothing else.
No, that just mean you do not understand the proper procedure on how these cases should be handled. Either the assessment of the antivirus software is a false positive, and then they should update their software not to raise it, because false alarms mislead their customers and lower the reputation of their software. Or the files indeed contain malicious code, and then GOG should not distribute it.
However, it's in the interest of both parties to clear the issue up to preserve their good reputation in the eyes of their customers. Therefore, GOG should have asked antivirus companies for explanation/evidence a long ago, and failing to obtain that (which I very much doubt would happen), issue a warning on the download page that certain product mark the files as infected but that to their best knowledge, they are not.
That would be a prudent and proper procedure on how to handle this issue.