title says it all-- basically just wondering if anyone has a solid indication that the DLC will arrive on GOG on the 31st?

I don’t believe it will be released on the 31st, no. The way I heard it explained, because Chucklefish is essentially a third-party seller, GOG would need to reach out to Chucklefish and negotiate some sort of pricing agreement, as opposed to the other way around…and there’s a way to sort of gauge interest by users voting on it through the GOG Community Wishlist.
I already created a wishlist for it which you can vote for…I’d post a link to it but I’m not able to post links yet. :/

Chucklefish said "In terms of other platforms where the Eastward base game is currently available, the dev team are looking into making the DLC available and we will update the community as soon as we have more news!"

(Source : X/Twitter)

The Octopia DLC has arrived.

https://www.gog.com/en/game/eastward_octopia