timppu: GOG's DRM-free practice is a honor system, where they expect that you don't pirate nor sell the games, even though they are not technically trying to prevent it. If you don't honor the system, then shame on you, you are obviously then not a honorable person, but something between an amoeba and that green thing you scrape off the shoes.
^ Minus the insinuations on a person's character, this.
A system with a lot of freedom only works if users of the system use that freedom responsibly, else it collapses.
If you really like DRM-free, then don't turn around and give (or sell) a copy of your DRM-free games to your pals and acquaintances, because this will make a DRM-free model a lot less economically viable on the seller's end of thing.
There exists no stable system under which the games are DRM-free and you can abuse that freedom to give away (or sell) the games.
You have a choice, either you respect this or you get heavily DRMed platforms like Steam.
So yeah, each time you give away or sell your DRM-free games, you are giving the DRM-free model a gigantic kick in the nuts. Don't do it.