Overview page: https://www.gog.com/en/anniversary
Direct sign up for Galaxy Linux beta: https://www.smartsurvey.co.uk/t/YEF5EV/
Sure, and were also assuming it works at all. What's your point? It'll almost certainly have something like this or there's no point in them making it at all.
Regardless, it still doesn't even rely on them because, like I said, you can do it yourself from the terminal. You could just write a simple script to launch the game and they'd run. Proton exists. It isn't a Valve thing that they have to remake.