I'd be interested as well. Certainly being DRM-free GOG.com's content typically ends up on torrent sites, but I haven't heard of it the other way around—though there are of course various examples of pirated content appearing in officially published downloads elsewhere I've read about and even experienced first-hand. One that comes to mind was 90s hip-hop act De La Soul generously releasing their discography for free during Valentines Day a few years back to promote a new record, which from some ID3 tags mentioned the source were pirated copies.
You have to wonder when an artist finds it easier to copy the albums from such sources than from the collection of their own.
I can't find the article I read this from, but a few years ago (back when GoG was 'Good Old Games'), there was an article about how license holders often didn't have any of the actual game material, they just happened to own the license to them through mergers / aquisitions / bankruptcies / whatever. So GoG would have to track down old published copies of games for them (either through second hand shops, or eBay, or the internet) and then restore them (using a combination of internet / open source tools, or their own work), in order to sell them on their store.
Presumably, if the internet 'pirates' had not done the work of backing this software up and preserving it, a number of GoG's titles simply would not exist today in any form, and could no longer be sold (even if the license holders wanted to)
I would imagine downloading a 'pirated' copy of the work is not piracy when you already hold a valid legal license to that work, by definition.
I didn't devote much time, but about half of the 10 manuals I looked at quickly are of horrible, almost unreadable, photocopies. One thing I did find is that some games, like Arcanum, use "warez" copy protection cracks.
Can you give one or two examples of this?