Perhaps there will be a way to redeem the key through GoG instead? I know nothing about such technology!
Not promising anything yet, but we'll look into how to do that best. We wanted to use Steam to give folks achievements and cloud saves (we're not interested in the DRM features, just its streaming and gamer stuff) but it may be possible for us to support DRM free on the disc, but still give fans of Steam access to those things.
Again not promising that, but that's how I'd ideally like to see it handled and we'll see what we can do!
Since the boxed copy is in addition to a digital copy of the game, I don't see why the disc version should be the Steam version.