Technically that also applies to Steam, since you get a digital good available at the moment of purchase for permanent offline download to an external storage, just copy the game folder and you’re done. It would be the equivalent of a music store place downloading mp3s (and the equivalent to GoG would be selling an .iso to the music CD you can burn whenever you want or an installer that extracts the mp3 to a folder).

If the game itself has DRM then that would also apply to GoG (yes, there are games with DRM on GoG, there’s just proportionally less of them).