Overview page: https://www.gog.com/en/anniversary
Direct sign up for Galaxy Linux beta: https://www.smartsurvey.co.uk/t/YEF5EV/
The GOG Galaxy UI is beautiful. This sounds like it would be great for conventional Linux desktop users (or Bazzite) if they're not using Epic or Amazon games. I'm happy with SteamOS though, and GOG Galaxy would be a hassle since it launches into gaming mode. Like most people, I'll probably stick to Heroic for the Steam launchers, but more choices are good!