So the reason was found out by a cracking group called Plaza
. Which for the first time actually cracked a Gog Galaxy game by emulating Ghost a protocol used by the EXE to wrap steam calls into galaxy to activate DLC.
Here is the basics:
This is a very interesting case. Square Enix/eidos gave GOG the unprotected binary but they left all the Steam stuff intact, probably because Square Enix didn't want to waste any of that sweet MTX money on dev work. So GOG had to add a platform bridge called Ghost to the game. It's a wrapper that translates the Steam calls and redirects them to GOG Galaxy so the DLC gets unlocked. This brings us to the unique situation where a GOG game, for the first time ever, actually needs a crack, which is essentially just a normal Steam crack without custom platform bridges. Enjoy!
Thats directly form the Cracking crew that popped the Ghost wrapper bridge developed by GOG. Hope that helps everyone know whats going on and why. Blame the Square Enix/eidos, but also blame GOG for not telling us.
All GOG needs to do is say: The below DLC will only work with Galaxy installed, that should cover their butts. The rest of the game functions without issue at all. I would love to see them actually fix it but i doubt they can. unless they dig into the Plaza crack and copy what they did.