I got a coupon for DLC to Age of Wonders Planetfall. I don't have the game on GOG, but on Steam.

Apparently the program that evaluates your library can't distinguish if you have the game on GOG or not.

I reported it through support.

That is odd indeed. That would suggest GoG is sharing data with Steam which I never agreed to..
The message I received reflected I don't own Planetfall and cannot purchase the DLC alone.
I have never purchased on Steam and never will.

I also got that mail with the discount code and own the game on neither platform.
This appears to just be a weekly newsletter kind of thing and has nothing to with GOG sharing data with Steam.

Speaking of which - as I can not use it myself, here's mine:

ZWYQ7A99B271D89273

Every Y is a G in reality.

_________
Edited a minute later: Removed spoiler tags - as they don't work.

It's the standard newsletter discount. Everyone who gets the newsletter gets that same discount. It has nothing to do with your library, wishlist or library on other stores.
There's a thread where people post the codes they don't use.

Thanks for the clarification. This was the first time I received am ad for a code I couldn't use.