I'm glad you got your issue resolved. I agree that the nightlies should be available here too and I am disappointed that they are not at the moment. The same thing happened with Don't Starve Together but, Klei made right in the end by giving us GOGlodytes a Steam key.
It's nice for devs that Steam has a lot of features built right into it but, when devs opt in to using them instead of developing their own solutions and then distribute their product outside of Steam, it sucks for the people who then buy them. It's pretty irksome in so that you are either locked into using Steam or occasionally having to deal with situations like these where the game you bought is lacking functionality. Some day Galaxy will save us or, at least provide an alternative whose problems come in a different flavor.
We can be mad at Steam for trying to monopolized digital games distribution, we can be mad at Chucklefish for relying on a third party to provide access to the nightly builds, we can be mad at GOG for providing a product with less than full functionality or we can be mad at any combination of the three. Confronted, GOG would apologize (evidently), Chucklefish would apologize (you can call them a lot of things but, malevolent would not be one) and Steam... Well, Steam's just gonna keep doing it's thing.
I would be surprised if GOG and Chucklefish don't find a solution to provide the nightlies on Galaxy, ever. Galaxy does have the capability to provide multiple versions of a product. The problem is that with Steam, devs can push updates directly to the client whereas with GOG, the devs send the updates to GOG, GOG checks to make sure they work and when they pass that QA, GOG then push the updates to the client. The end result being that GOG is slower to update than Steam but, has fewer headaches as GOG try to catch any issues. Neither solution is perfect but, that's what they have been but, we should expect GOG to streamline the update process now that they provide 'Games In Development'. Because games that are "In Development" but, don't receive updates are really just incomplete games.