ConsulCaesar: Don't Google ads use your browser history to select what ads to show? If you do game things in the Internet, you get game ads. Or even better, you've been visiting GOG during the sale, so they don't want you to forget to buy games.
If you use chrome, perhaps. Otherwise they may try to use some tricks to try and approximate your internet history (cookies, web beacons, google analytics, like and share buttons...) to find out what you have been seeing.
The Youtube videos embedded in the game card pages? Google know you're at GOG.
But it does seem that advertisers don't know how to distinguish an existing client from a prospective one.