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.
'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.'
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.