Same problem, here. GoG version.
All checkbox are greyed, same for the "install" button on Galaxy. I tried to install them with the .exe of the website, but the installer just get stuck and never finish the job. I contacted the GoG support, but so far their only response is : Are you sure that the game DLC are not to be activated in the game...
First time I am so disppointed by GoG team. I should have choose the steam version ^^