Just did a GOG survey that focused on the idea of a paid membership option on GOG. Seems they're determining what people would be willing to pay extra for. Some of the options were
- a tool for backing up offline installers
- ability to install previous versions of a game
- extra insight into the preservation work they're doing.
- voting rights on games to bring into the preservation program.
And others that I can't remember.
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.