Sort of (from the crowdfunding FAQ):

Q: Will the game be available through GoG?
A: Yes, most likely.
We're already in contact with GoG and they told us that the game is very compatible with their audience. But they still need to playtest the game a little bit before adding it to their platform (curation process).

As always, can't hurt to vote in any case:
https://www.gog.com/wishlist/games#search=realms%20beyond

Just wanted to note that sale of RB on GOG has been confirmed as you can see there's a RB page here.

It is going to go for sale on Steam Early Access next year (estimated first half of 2021). Currently it is not planned for them to sell EA version on GoG's "in Development" though I suppose it's possible that sufficient interest could convince them it would be worthwhile.