You will have the option to pick Steam or GOG for your download version. The physical copy is just a physical copy and not tied to any particular download agent.
That does nothing to answer my question.
I'm trying to find out if the "DRM-free download" can be switched to Steamworks, as well as whether the DVD copy (which is listed as "DRM-free") can also be set to Steamworks.
I had a similar experience when I decided I wanted to confirm my order ($65 tier), and found the list of items included in the tier to be not exactly the same as the Kickstarter list. I decided to send off to Obsidian support as a result.