Earlier quoted context omitted.
Doesn't DRM-free literally mean that you get a full local installation without any reliance on online licensing or any other digital restrictions?
Sure, just like the olden days. But you still need a license key or equivalent - for an offline check. And in the case of an app store that is handled by the app store, you never get or see the key. So if you are banned that doesn't help. And even if you had a key you still don't have access to the install media. You could try to extract it from your drive but there is no guarantee that will work on another computer…
A GoG game comes with a single installer executable (single as in it is the same for everyone, no built in license) that and the resulting game it installs can be freely copied and used on as many machines as you like without any requirements. For example you do not need to have an internet connection or be running the GoG store application.
So if you have downloaded and backed up the installer of the game you bought on GoG then for all intents and purposes you own this game. It does not matter what GoG does from this point on. You will always be able to use it.