I got a DRM free + steam key version of Sunless Sea from the humble store a while ago - I’ve been playing the DRM free version which I download.
I want to get the Zubmariner expansion but it looks like it’s only available as a steam key. Meanwhile GOG won’t let me buy DRM free expansion because I didn’t buy the original game through them. What gives??! I want to give you money for a thing yo but literally can’t buy it.
If you’re willing to use the Steam version of SSea instead, you should be able to purchase the DLC key through the Humble store without issue. The game stores all save files in the same directory regardless of whether you’re using Steam or the DRM-free version, so those would still load just fine.