Well it's not going to be the same engineering challenge as it was for Valve, because they only need to integrate proton, not develop it. If proton works on Lutris (via umu), an open source project with no corporate backing as far as I'm aware, surely CDPR can at least attempt it. This is probably the best time to do it, too. SteamOS has been well received and is likely to end up on even more handhelds, and Windows 10 is nearing its EoL. If GoG is one of the first storefronts to allow its users to play outside of windows it might generate a lot of positive sentiment in the community, just like they did with their anti-DRM stance.