timppu: GOG games don't have any technical limitations (DRM) about how you can use the game once you have obtained it. They have legal limitations (ToS or SW license agreement) though about what you are allowed to do with the game purchased from them.
Orryyrro: Except the reason you can't legally sell them has nothing to do with the ToS, it has to do with the fact that the game is tied to an account, which is an extra thing you would have to sell in order to legally sell them, hence DRM.
DRM has nothing to do with what you can legally do with your purchase. DRM refers only to the technical means to restrict what you can do with the game you have obtained, usually trying to prevent the same copy being played on several PCs simultaneously (CD check, codewheels/manual checks, online authentication etc.).
There is absolutely no DRM in GoG games that prevent you from making millions of copies of a game you have obtained from GoG and selling them to everyone (without selling your account). However, that is still not legal.
For example Steam is different, because there you basically need the account to play the games. In GoG you don't.
Orryyrro: Again, yes they are tied to an account, because it allows you to download them again as many times as you want, the only way a game could be DRM free would be if you could remove them from your account at will.
Being able to download the game several time from your GoG account is just an extra service, not a requirement in order to re-install and play the game, even on a different machine. Thus, it has nothing to do with DRM, which is about technical restrictions about what you can do with a game
you have already obtained.
GoG games install and work fine even without an account, as long as you keep a local copy or copies somewhere.