I've had GOG giveaways for years and use unique emails. GOG has never sold my email. I'm pretty sure they are doing this so they can send you emails about deals, to try to convince you to buy more games from them.

They mention this in the dialogue when you click the button but you can immediately go to https://www.gog.com/en/account/settings/subscriptions and just uncheck the option. I've done this so often on gog I've just gotten into the habit of immediately opening that page and unchecking it, basically every time you get a free game from them it checks the promotions option

Yeah maybe but you get the game for „free“ it’s the same as using google search 😅