It isn't a zip file (that is Itch) but an installer (you can't easily install it just as a non-admin user on Windows, although on Linux it does install under the current user). They had a .tar.gz for Linux at first but now use a self-extracting installer there as well. A few years ago they said (didn't save the link) that only something like 5% of customers just use the offline installers (and I'm fairly sure not all that many more ever downloaded them, although it might be higher now after a few update issues in Galaxy). It is unfortunately not uncommon for games released on GOG to not have Linux versions available elsewhere. CDPR games have had Galaxy-exclusive bonus items, which is hardly being friendly to offline installers.

I don't have any inside knowledge but there is clearly an internal battle at CD Projekt (and their investors since they are a public company) between those who want to print money doing the unethical stuff that other game developers and stores do and those who want a more customer friendly approach. They tried a more ethical online focused thing with GWENT (that ended up partly under GOG due to relying on Galaxy and was a big reason they pushed for higher Galaxy use for a while) but it ended up not really making much money. Things like this California law are great to help support the availability of DRM-free games.

Also, GOG has around 6500 games now and I'd be supprised if they were involved in getting more than about 100 of them to work on modern systems. Galaxy has been around over half the time GOG has been in business (as an online store, not counting the early CD Projekt days). You are thinking of the early days of GOG but they are quite a bit larger now and CD Projekt as a whole is much larger now. I still think they are the best option to support DRM-free games but they are not the same as when they started (not only in bad ways, the refund policy is great now).