Klei Entertainment announced "Don't Starve: Shipwrecked" is coming in fall 2015.

See here:
http://forums.kleientertainment.com/topic/56660-dont-starve-shipwrecked-coming-this-fall/

The question is if or when we can expect to get the add-on here on gog.

This may be an old thread but I think it's still very current and I don't see any other threads on this matter..
My thoughts are that Klei Entertainment absolutely abandoned the DRM-free market and GOG along with it in favor of Steam.
Hence you see neither DS-Shipwrecked or DS-Together on GOG.

We didn't see the two that are already here until they were at release status. Why do the two you mentioned have to be here during Early Access, exactly?

In which way is it early access? I can understand don't starve together to a degree but I don't see them actually adding anything that really changes the early access aspect, all that's been promised is "base game integration" which is already there.. And "more characters" which seems more like a DLC addon or an update.
And why else would Klei offer don't starve together copies for Steam and not GOG?

Shit, my mistake; I swear I saw Shipwrecked as Early Access. Anyway, I got a response in regards to it being here.
[start]
'As with Reign of Giants, the final release of Don’t Starve: Shipwrecked will be available for the standalone client. That being said, we do not have any information to share about it being on GOG at this time.

When we do have information to share about that, we will be sure to shout out about it.'
[end]
I can't answer your specific question on DS Together, sorry. Speculation does nothing to inform, However I did read that they used the excuse that Steam's API was needed for it blah blah. This was a while ago, but I don't see their stance changing.

edit: lol, now I see it is in Early Access... so I guess I can't answer what you're asking on that either.

Back in the days when Don't Starve Together was just announced and released, those who already had a copy of the singleplayer version of the game could get a free copy of DST, but it was obviously not available for GOG, so the GOG customers had to contact Klei about getting a Steam key instead. Hence my claim of Klei abandoning GOG in favor of Steam.

I don't really expect DST here, honestly, because it uses Steam for its networking. It's possible, however, since it does have a LAN option, so theoretically they could add in a direct-IP connection option, or simply offer it here as LAN-only.

Shipwrecked, though, I fully expect to arrive here on GOG once it's finished. Shipwrecked is still VERY MUCH a beta, however. They aren't even finished adding all the characters into it yet, and there's a lot of gameplay that's still quite unpolished. I, for one, am not desperate to try the beta, so I'm content to wait here for it to be done.

It's here at GOG now:
https://www.gog.com/forum/general/release_dont_starve_shipwrecked_9781d/page1

Better late than never ;-)