advowson: I think it is likely that this is just a typical GOG-related delay. The DLC requires the latest Sins version (v1.93 released Dec 14th) to use it. That means there's not much point in selling the DLC here until GOG offers that version for download to people who bought Rebellion here. Steam users who have v1.93 through Steam could use the DLC, but if they bought the base game through Steam, they would probably buy the DLC there too.
As of now, I only see v1.921 offered on the GOG game library page. GOG is not known for speedy patch delivery, and that's probably especially bad during major holidays. I expect that this DLC will become available here at some point, but almost certainly not before GOG makes Sins v1.93 available.
As GOG has clarified
earlier this year, it's mostly up to the developers and publishers to roll out updates for GOG versions of their games. They could have simply used GOG's
developer portal in order to access a build delivery system that is similar to Steam's own content distribution system.
Unfortunately, we don't know what's holding Stardock back from releasing the update and the DLC on GOG.