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).
I had a think about this scenario and I think that if Steam was going to present this argument they would need to document and support this workflow. At the moment the fact that it sometimes works is more of an accident than anything (essentially it’s all just files on a disc and sometimes the files still work if you move them somewhere else).
But if they document that you can transfer the install data to another location, and identify which titles that applies to? Then I can see a reasonable argument that they qualify.