So apparently Trepang2 on Steam had a $3 survival DLC release:
https://store.steampowered.com/app/2615910/Trepang2__Survival_Mode_DLC/

Do we have an ETA window on when this DLC will come to GOG?

Whenever Team17 starts to care about GOG again. The devs can't do anything about it sadly.

Yes, I have posted to them over at the steam forums, and also sent an email to them regarding the same. Let's see how long they try and ignore it for. Looks bad on them as a gaming company to do these kinds of things to their customers, yes. They have some making up to do IMO.

Edit: a user uploaded a screenshot of a Dev's tweet i think on another topic here on GoG's forums. They said there might be an update in the works for maybe Jan 2024. Cross our fingers and be patient I suppose.

Yeah this kinda stinks. I really love the game and would like to buy its DLC and play the new combat simulator maps. However, we haven't received any news or posts recently mentioning an ETA or anything regarding updates on GoG. I'm getting concerned the GoG edition has been abandoned. I sure hope I'm wrong.

Seems to be official now. Let's hope they're able to deliver in January.

let us also hope that they do what they should be doing from now on with the game. Updating it on both Steam & Gog at the same time, leaving the game for months without support on here is rather inexcusable, and has been done by too many companies too many times to be forgotten. So it would be nice now if because they have promised officially to do the update in January that Steam would get a new patch, and they would also bring Gog FULLY up to date at the SAME time or within a FEW DAYS!

Any news on additional content for GoG version?

still completely quiet here or on steam forums. So now they are getting MORE posts pointing out how underhanded their fake promises were etc. This will affect sales. So, all they have to do is some actual work which would take all of 5-10mins of labor. Really just find an employee who knows how to do a basic upload to the storefront here. It should be pretty easy when you have enough technical ability in the first place to both create and market a game!

Hey guys, just a quick update: the publisher has contracted another studio to work on GOG updates. The latest patch and DLC are now going through final testing before it's going to be available to everyone. Almost there, really sorry for the delays and thanks so much for your patience through all this. Hopefully the next patch will go more smoothly now that we have another studio working on it.

Solved I guess, haha