There was a screenshot shared on the Heroic Games Laucher sub-reddit some little time back, in their regular news posts (which all seem to have been deleted now) which was between the moderator there and one of the Heroic devs. It showed them chatting about the money Heroic gets when you buy a GOG game by using Heroic, or use their link.
https://heroicgameslauncher.com/donate But here you can see that if you use their link there:
"One-time. Use our link when making purchases on GOG and support the project. Cool right?" it will give Heroic a % of the sale to help support their development. It is the same when using the Heroic Launcher itself too, it just uses that same link and opens in the browser!
But I will try reach out to that moderator and see if she will share that screenshot again so I can share it
In the end, it's so nice that GOG have this set up with them! It can't be much of a cut, but still great that they're officially supporting it in some way!