There is nothing special about this. They are also offering other apps to download, e.g.

Spotify:
https://store.epicgames.com/en-US/p/spotify

itch.io:
https://store.epicgames.com/en-US/p/itch-io

Brave browser:
https://store.epicgames.com/en-US/p/brave


And there is already a thread about it:

https://www.gog.com/forum/general/gog_galaxy_featured_on_the_epic_games_store

I wonder why gog hasn't brought galaxy to the windows 10 / 11 store. Epic games is in there. Would probably make more sense.