So, GOG always gave away the DRM free version of Witcher games to Steam owners of those titles.
The way it was handled with Witcher 2, well, a lot of people exploited that to give away their GOG copies of the game.
But now there's Connect, so it would be possible giving away W3 Goty to owners of W3+season pass on Steam?

Well obviously that wouldn't be fair to the other platforms.

Since the Steam api that GOG Connect uses won't read DLCs, the answer is most likely no.

What do you mean? GOG did this with other Witcher games too.

After a couple of years. Give it time, it'll happen eventually. After they're done milking the Witcher 3 for most of it's profits and letting you add it to GOG will make a nice publicity stunt.