Download the offline installer and now you have possession.

Further, because GOG enforces no DRM or mandatory sign-ins, so that means that you also have control over that copy of the game - you can install it and play it however and whenever you see fit.

For GOG games were you downloaded and kept the offline installer, the only way that the rightsholder has of stopping you from installing and playing that game is to sue you, which is:

• Almost certainly not worth it for then due to the cost of taking somebody to court for it versus what they would gain if they won.
• They actually have to have an actual legal basis for unilateraly changing the licensing agreement with you and hence be allowed to block you from continuing with the activity you have a license for (installing and running that game), which they almost certainly do not have in pretty much the whole World since EULAs aren’t valid there (they’re considered an attempt at, after a sale is completed, forcing unilateral changes to the implicit contract which is the sale, so have no legal validity)

With other stores they can just force the store to take down the game and now it’s up to YOU to sue THEM to get restitution and the “not worth it” is now something that goes against you (unless you have access to a Small Claims Court in the jurisdiction you’re in, are you really going to sue them for a 50 bucks game?)

In fact, physical media is worse than digital GOG games, because the former can have phone-home DRM or mandatory sign-ins so you have possession but not control.

PS: However if the point you were making was about resale, then it’s a valid point. Mind you, physical media with phone-home DRM or mandatory sign-ins can also be de facto unsellable if that’s what the game maker wants.