I was advised to sign up for the newsletter to receive alerts of free games on GOG. So I created a gog account and, in Settings>Subscriptions, signed up for the "Promotions and hot deals" newsletter.

This morning, I got my first email. It's the one where Arkham Knight and Arkham City are 79% off.

1. How often do we get email alerts of games that are 100% off (free)?
2. Is there a site/webpage that lists a history of games that were 100% off at Gog?

Giveaways are usually during seasonal sales, holidays or some other special occasions. Few times a year.

You can see past giveaways here:
Part 1: https://www.gog.com/forum/general/list_of_free_games_on_gog/post5
Part 2: https://www.gog.com/forum/general/list_of_free_games_on_gog/post262

I recently removed all notifications because of all the junk they keep sending that i don't care about.

You might get a message but if it's a 24 hour giveaway you might get said email after the giveaway is over...

If you go to the main page, you'll find a link to games that are permanently free:
https://www.gog.com/en/partner/free_games

In particular, if you claim GWENT, you can then also claim The Witcher:
https://www.gog.com/gwent-welcome-bonus

If you participate in the forum with worthwhile posts, you can qualify for the community giveaway:
https://www.gog.com/forum/general/the_community_giveaway_gog_edition_redux/page1

Good know, I'm new in Gog. ¿You need pay 15 DLC Gwent and Gog give you The Witcher free?
Sound bad busines

Same here. Besides, I had very little benefit out of their umpteen clickbait emails. When they couldn't even be arsed to write what the specific offer was, I removed that crap from my inbox.

If you have signed up to GoG solely to receive free games, you will be sorely disappointed. GoG is a business that sells games to make money, if they can't do that, they won't stay in business very long. There are quite rare occasions when games (or a game) are allowed to be claimed for free, which is usually done in collaboration with either the developer or the publisher or both. But as said, they are few and far between, and usually happen when least expected...