Well, there are a bunch of files scattered through the install folder named DLC02 through DLC06. DLC05 seems to be the Leaders Pack. You could try deleting them but I don't know if that would fix the problem or just break the game.
SImply uninstalling the DLC pack broke my game, and I had to reinstall it completely, so I'm not sure it's a good idea, but may be it could work. But, GOG should give us separated installer for each DLC, this way we could easily choose which one we want to install in a clean way.
If you install it manually it already seems to be a separate file.
Non-galaxy version of Legendary Heroes Ultimate has two .exe : one for the main game, and one for the "DLC pack", that you can simply instal or not (and that's all, no choice at all unfortunately for now).
But thanks for trying to help :)