If they were to force a TEE based DRM like Widevine L1, it would likely cause significant issues as there are a considerable amount of devices that don't support it (for example most PCs).
If they were to use software based DRM like Widevine L3, it would be easy for enthusiasts to crack and the tools for doing so would just get much much better.
With the Steam Deck getting more popular and more SteamOS handhelds on the way, it has never been a better time for game companies to support Linux. GOG does already sell some games that have Linux support, they just don't have a convenient way to download and install them.
GOG galaxy appears to use CEF and Qt, as well as some parts (such as plugins) that use python. All of those are cross platform. So I doubt it would be incredibly difficult to port to Linux. The fact that there is already a macOS version indicates that it can be made cross platform and can run on Unix-based systems.