According to this article:
https://www.origo.hu/techbazis/20201...ition-gog.html
you have to install GoG-Galaxym go into the "recent" folder (the basic folder when GoGalaxy starts), and (if you don't own the game yet?) you'll have a banner to click on, which'll ask you to subscribe to the GoG-emails at least for the period you receive the game (then you can unsub).
Btw, GoG's newsletters are not that often, and many times ocntain additional discount options, so it's totaly fine to leave it turned on.
NOTE: I do have to game already from a previous promo, so I cann ot confirm the article, or at least can tell the banner is not there if you already have the game.