Most games have their latest official patches applied, others have official patches + fan patches if any are available.

The latest official patch in the vast majority of cases.

Thanks for your answer. Is there a way to rollback updates on GOG to get the original game files, like they were shipped at launch?

Not that I am aware of, you will probably have to track down a hard copy of the game if you want that.

Just to confirm, this is unfortunately not possible. One of the main things I dislike about digital distribution. You can try manually reverting by finding the earlier patches online and overwriting the files when you download from GOG, but there is a high likelihood that the game just won't work, depending on what GOG did to make the game work and the way the patches themselves work.

Or as mentioned, hunting down a box.