In not linking reddit, which has way more details about how it went about.

https://www.theverge.com/games/945088/gog-apologizes-email-nazi-symbols-the-end-of-the-sun

In email marketing, you typically send emails in batches. Sometimes for A/B testing, other times to avoid spam catchers/overloading the system. Or sometimes for geo location purposes (like everyone gets the email at 8am, so it batches it to time zone)

The GoG reps said they sent it to a small batch and after the call out, stop the email sends and fixed it.