JakobFel: No, it'd be DRM-optional. You wouldn't HAVE to opt in to the trading program but if you wanted to trade them, you'd have to route through the client. That's optional, ergo DRM-optional. Furthermore, it would be as simple as a flag for each account. If they download the game via the offline installers, they're locked out of the trading program. It wouldn't be hard to do from a technical perspective, just from a legal and logistics one.
AB2012: You seem to be misunderstanding how modern offline installers are created. In order for any store to allow reselling games, it requires DRM to enforce the removal of the ability of the seller to play, ie, if you want to limit resale to Galaxy-only downloads, those Galaxy downloads will require an ownership check upon every game start (handled by Galaxy exactly like Steam DRM). Which is DRM. You claim we can pretend "it isn't DRM" if there's some "separation" between Galaxy vs offline installers but there really isn't any separation and hasn't been since 2014 where the only truly Galaxy-free offline installers we had were the pre-2015 ones with the older
nameofgame_2.0.0.x.exe naming scheme. Since then, most offline installers
are built from the Galaxy build (hence why they include galaxy.dll, etc, (
and often refuse to start if you delete it) despite there already being zero functional need for any Galaxy code in offline installers). Unpack an installer using InnoExtract (GOG use InnoSetup to create them) and they aren't even stored as files, they are "Galaxy streams" that get rebuilt into files. This is the cause of incidents like Saints Row 3, Deus Ex:MD DLC not unlocking, etc, the offline installer build *is built from* the Galaxy build.
So it really isn't "technically possible" for offline installers to be "completely separate" from Galaxy given GOG have spent the last 5 years going through the whole catalogue and Galaxifying the offline installers as much possible thus removing the same
"build from two sources instead of one" separation of builds that you would require for your idea to work. And they definitely aren't going to go back and redo the entire store's back-end to permit re-sales that actively leaves them financially worse off, and publishers won't agree to anyway (for the same reason existing DRM'd platforms like Steam could already technically allow it, simply don't want to).
If they added a flag system to each account, it'd automatically mark whether or not a person has downloaded the game's offline installers. It wouldn't need a constant check because if they download the offline installers, they'd immediately be locked out (in my idea, that is). The only possible problem would be the fact that you can still play Galaxy games offline, in which case they'd need to modify the Galaxy versions to have DRM capability, which is precisely why I said the idea would never happen.
Again, I'm not saying I want this, it was just me giving a possible idea on what a trading system could look like on GOG. I'd never use such a feature either way, and I realize that it has flaws that would be questionable at best, I was merely brainstorming a possible starting point for the sake of this thread. Not gonna happen either way, though.