*** UPDATE ***
Bad news so I edited the first post :(
I'm quoting the dev directly with its permission:
"I don't have an account on GOG so maybe you can quote me on this:
I work with a publisher for the non-Steam stores and I expected before release that they would be able to get me on GOG. Recently they informed me that GOG rejected my submission (I do not know on what grounds) and that they were going to try re-submitting. I trust the publisher. They have gotten the game on almost every other store so I don't think the issue is with their submission. I am going to remove the GOG button from the website for now to avoid further confusion.
If I were to release on GOG, it would be with a DRM-free version only. This is mostly because the work required to integrate the Galaxy API doesn't seem worth it when GOG users don't want it."
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Here's the wishlist entry in case you want to upvote it:
http://www.gog.com/wishlist/games/pony_island