Unfortunately, we aren't distributing GOG keys to backers. But, the build at GOG is the same as the original DRM-free release, so you get the same version and updates.
Well that's a darn shame to hear :(
"Q: How will I get the game?
A: Backers who pledge at least $15 will be offered Steam codes for PC or Mac as well as DRM free versions for PC, Mac, and Linux."
I can see how it was easy to assume that "DRM free" plus it being released on GOG would mean that backers got a GOG key.
Can I send you guys back the shirt for the $70 pledge and swap it for a GOG key instead? One would think with such a successful Kickstarter campaign that arranging backers to get GOG codes(a preferred convenient method of DRM free downloading) just as Steam users have Steam codes would have been the...considerate thing to do. It certainly makes for easier PR than the above quote.