hello same Problem no "more" function in this game
First, make sure you're using the website - not the Galaxy launcher. (Go to https://www.gog.com/account
in your browser.)
Click the picture of the game on your shelf (not the downward arrow at the bottom right corner, but the picture of the game). A card should pop up with the game's name. Under the name you should see options for System, Language and More. Click More > Serial Keys.