PowerPCx86: my idea is that every gog installer should have a unique key associated with the owner account
You are not the first one to think of that, it has been discussed in the past also in this forum.
I have also mentioned in the past that, as a person who buys games on GOG specifically for DRM-gree games, I personally would be fine with "watermarking" of GOG games, where the game might include some kind of hidden "watermark" which could connect the game to the original purchaser, with some kind of unique code.
The "watermark" could be e.g. your GOG user ID, so seeing it will not identify you personally, except if you are GOG staff who can see which GOG user has which user ID. So 20 years from now when GOG store has stopped operation, that watermark will not really tell anyone anything anymore. "Oh, this GOG game was originally bought by the GOG user ID "78923479792874877292374987398742798237984747298". How cute. He could be Donald Trump for all I know."
So the watermark would not technically limit who and how people can use the installers, but they would be an incentive for people NOT to share their purchased GOG game installers to the world, or even their friends.
However, I don't know feasible such watermarking would be, without being very easily removed or altered by hackers, or by anyone by some simple third-party tool or instructions on the net. But if it was possible, I personally would be fine with such "watermarking", as I don't share my GOG games out of my household (and I let no one else access my installers; if my son wanted to play some GOG game, I would install the game to his PC).
So I think the best way for GOG to combat piracy while remaining DRM-free is simply by offering better support for their games than the pirates do, like constant updates etc. And if there really are some sites online that offer GOG game library, naturally go after such sites.
SpeedBo: This isn't just a bad idea, it's a terrible idea. You clearly don't understand what DRM free means. Also if GOG did this, I for one wouldn't bother shopping here again. What's the point if you're just going to add DRM to the game installer? Might as well use Steam.
It is not DRM because it does not technically limit how and by whom the installer can be used.
EDIT: Well, someoone might argue "it is DRM because you can start the installer only with a certain username and password", but that would be a rather useless method of "DRM" if that username and password could be simply included with the game in a readme file.
The original idea of "watermarking" the installers would not have such extra step to enter your user ID and password, but that each purchased installer would have their unique watermark, an ID.