Does anyone know how this DLC will work with the game purchased elsewhere? I already have Outlast from the Humble Store, and would like to purchace Whistleblower from GoG. Humble Store gives both a DRM-free download (like GoG), and a Steam key. Will GoG's Whistleblower work with Humble Store's download? What about the Steam version?
I can't find the thread where someone was asking this same question about non-GOG games and GOG DLCs. In short, the answer Judas gave was no. Fundamentally, it should work but there is no guarantee that it will since GOG creates their own installers for their games. It is a gamble. Installing a DLC on a non-GOG game voids the 30 Day Guarantee, so if it doesn't work, you may just have wasted your money.
EDIT: Here is the thread in question. It was in the Blackguards forum. http://www.gog.com/forum/blackguards_special_edition/dlc_from_gog_with_retail_version_of_the_game