I had some of the marketing email options enabled on my account (apparently... I don't remember enabling anything but wishlist notifications).

During June, I received 29 emails from GOG.com. At that point it's just noise. I unsubscribed to everything now because it's simply not worth it to have half of your inbox dedicated to GOG marketing. The thing is that I would be interested if it wasn't for the extremely high volume. Consider introducing a monthly/weekly digest mode

Hence why I only have emails on for wishlist entries and dust gathering.

Every time you claim one of the promotional freebies during a front-page giveaway, you are agreeing to receive marketing communications from and about GOG, both directly and via their "Trusted Partners". It tells you this in not-very-small text right next to the button you press to add the free game (or goodie pack, or whatever) to your library. I'm gonna go out on a limb and guess that's why you were signed up for more kinds of e-mails than you remember having enabled.

Yep, you can get a lot of emails from GOG if you don't select the right notification settings.

I have all disabled except for sales and new releases.

I used to also have Wishlist games enabled, but because I have too many games on my GOG wishlist I needed to eventually disable that. I use a program of my own to keep tab on Wishlist price changes now, which is superior because it keeps a history of price changes and so I can do proper comparisons, and also study how things often work.

As has been said, if you grab a free game on the main GOG web page via the CLAIM button, you are automatically opted in for emails and adverts from GOG's marketing partners. So if you don't want those, it pays to immediately disable that subscription after claiming the free game. You can even use a script created by TheDcoder to do that automatically for you.