As the title suggests, Lutris is requiring me to sign in to GOG so that it can verify my ownership of the game. If my memory serves me well, I used to be able to simply click on the GOG version and then link it to the installation executable file that I downloaded from gog-games.com.
Is it trying to download the install files? That requiring a login makes sense.
It never used to do that before it just made you add the install files yourself. But yea that is probably what it is doing now.