Overview page: https://www.gog.com/en/anniversary
Direct sign up for Galaxy Linux beta: https://www.smartsurvey.co.uk/t/YEF5EV/
Valve also has the absolutely massive advantage of a significantly larger amount of income. It's like complaining that an indie game doesn't have the graphics that a AAA game has. Sure, it's great that Valve is contributing some amount to Linux, but they can afford to. It's also not purely to be nice either. They are expecting the investments to have returns.
I'm not going to complain that GoG took a while. I'm only going to be happy they eventually have taken the step. It's probably at no insignificant cost to them, and they easily could have just not bothered, but they see the market moving and are trying to get ahead of it.