is it possible to lose the de jure right to install the game in that way due to licensing issues on GOG’s end

Someone can correct me if I’m wrong, but my understanding is that no, you can’t. When you buy the game, you’ve obtained a perpetual license to install and play that game, similar to what you’d have if you bought the game on a disk. You can lose your ability to download the game, that isn’t guaranteed to be unlimited or perpetual, but installing it via the installer you downloaded, and playing it once you do, are forever. (This is in contrast to something like Steam, where you rely on their servers granting you permission to install the game, and that permission can be revoked.)