Luna you’ll be able to play every game that you already own on GOG (and that is also available on Luna). There’s absolutely no requirement to purchase anything twice

In other words, buying games on GOG will give you the best of both worlds – enjoying them on Luna’s cloud gaming service, as well as via offline installers or GOG GALAXY. All your DRM-free benefits of owning games on GOG will still be there, you’ll just have more ways to play your favorite titles.

It's the best way they could have done it, just an extra way to play with no lock-in. I fail to see the issue with this.