I kinda feel screwed over at this point as a kickstarter backer, for choosing the better platform.
Technically, I do agree, albeit not a backer, but I also try to understand. It's extremely easy to deploy software on Steam (compared to many other storefronts) and as a developer you probably prioritize the platform that generates the most (if not the vast majority of) revenue.
However the Halcyon 6 developers have always been very open and have supplied all of the platforms with patches - which unfortunately can't be said for all the developers out there which sometimes seem to see GOG as a "throw them game and forget" or "bargain bin" thing (looking at you, Vlambeer).
Honestly, if the strategy is to get the DLC out, supply a few patches, get bugs and balance issues ironed out and THEN send it to GOG so they have less troubles needing to supply (manual) patch after patch after patch, I'm actually happy with that - on a personal level (again, not a KS backer). It saves me the trouble of buying a game, installing it and then realizing I should maybe wait a few weeks for the first patches to roll in.
I can't say I am fully happy with the situation, yet I understand it. Honestly, if I were to just go "I buy on Steam instead", that would just cause such things becoming even more frequent and drastic.
I'd say bring it up with them (Mail, Twitter, etc). Who knows, maybe you'll get the DLC for free or they'll try to speed up the process.