This might be a case that can be fixed by keeping a certain file or folder immune to virus scans, but this is the second time I've had this issue with games from here.

I did notice the game now identifies itself as a Steam copy, Paradise Lost or not, which is confusing.

I don't own the game, hence I cannot check the file for viruses, anyway when I suspect a false positive, I usually send the infected files to an online virus scanner such as this one and let them double check the file for me with multiple antivirus software to confirm the original detection. I'm confused as well: what do you mean by saying "the game identifies itself"?

To add on to the good dr.schliemann's response, if Kaspersky has an option for it - submit the file to them for inspection so they can possibly re-evaluate it and update their detection database if they find it actually is identified as a false positive.

When you're in the Main Menu, in the bottom left it says: "Postal2: Steam-US-5023". Both the original and Paradise Lost have that message.

Well, I think it's a simple mistake of the developers: the 5023 patch was released on GOG and Steam at the same time and the game is DRM-free on Steam too, so they probably released here the same version they published on Steam.