The day it came out on steam, if I'm not mistaken, there was already a news post it would be coming to GOG. Was it widely publicized? No, it wasn't. But there was news available it was coming here.
Not to sound like a dick, but blaming GOG for not advertising is like beating a horse when it's down. They just do not advertise much of anything beyond their in house titles from CDPR.
It's like picking on the one armed man at the bar. Sure you can do it, and you might even feel good for a little bit, but you know it's wrong.
They just don't work that way, and that's part of the reason they're so far behind Steam and EGS.