timppu: It is legal already now to pass or sell game licenses you have to someone else.
I think this would be up to court decisions. There are principles like "Exhaustion of the rights", but also the practical reality of digital sales.
IANAL, but if you bought a game on GOG and made a legally binding agreement with someone to sell them that game (you give them the installer and swear to never use it yourself), chances are, this would have to be legal, no matter what the TOS say, because it's a principle of European law.
On the other hand, it's all splitting hairs anyway. DRM-free is basically built on trust in the first place. So if you buy a game for yourself, then sell it (under the assumption that you mean it) and possibly spend the money to buy more games - you already work within that trust system, because you could have downloaded that game from some torrent site anyway, and the guy who bought it from you could also have downloaded it from some torrent site, and the chances of being caught are really slim (GOG doesn't even seem to put much effort into shutting down those "free gog games" sites with direct downloads).
So in the end it's technicalities we're talking about. DRM-free is about the trust that we - the gamers - are willing to spend money on the thing we love, even if we could get it for free, and rather easily too. GOG and CDPR have proven that it can work this way.
Digressing starts here: So why is there still DRM? Of course many people in power in the industry still don't believe in living without copy protection is possible. But I think an even bigger reason is that DRM is not only about that, but about control and surveillance. They force people to use clients, not because it prevents piracy (most games are cracked day 1, some even earlier), but clients allow to watch what people play, how they play (achievements!), how much time they spend with what.
And now Galaxy... it's the same thing, with Galaxy 2 even across platforms. That's really valuable data. The difference is - they don't force us (nudging, sometimes even in annoying ways, yes) to use it. They trust us not to abuse the DRM-free, they ask us to trust them with Galaxy. But at least we have the choice. It's up to each one on their own.