Overview page: https://www.gog.com/en/anniversary
Direct sign up for Galaxy Linux beta: https://www.smartsurvey.co.uk/t/YEF5EV/
The good news is that Valve is not the steward of the PC ecosystem, even (or especially) on Linux. Everything they've contributed to Linux is open source, and they don't control it. Even if Valve disappears tomorrow, or enshitifies, there are tons of Linux distros and the packages are all public.