Great question. Early Access is out on Steam now but I wish I could buy Gloomwood here instead. Hopefully there will be an update about this issue soon.
Rudgary: Hopefully not the same story as most games, Midnight Fight Express had a release date on GoG and only came out on Steam for example. Can't remember if Gloomwood had a release date on the GoG store page though.
I've also noticed most early access games wait till they have more content before releasing here.
Edit: Grammar.
Early Access games ARE sold on GOG too. If ther is an impedment could be a exclusivity deal with Steam to release the Early Access there first...