As seen in Steam:http://store.steampowered.com/app/223220/

Not yet, but...

http://www.gog.com/forum/giana_sisters_twisted_dreams/remixed_mode/post11

That was only today....how did I not notice that?

Hehe, from the Steam store page:

"“The sheer amount of detail and work that went into the graphics do indicate to show you the sheer about of love for this fraaahhh-!”
– TotalBiscuit, The Cynical Brit"

Yeah, paraphrasing voiced review is always fun :D

That's not even a confirmation that it's coming though... it's just them saying 'if we get it, we'll put it up as quickly as possible' , which is common sense...

GOG got all the other updates and new levels for GS:TD so I'm pretty sure they'll get this one also.

I'll wonder what will happen with Rise of the Owlverlord though when it is released, if it will be here on GOG or not, because that's really DLC as I understand it.

GOG already sells DLC for other games, so I don't see why they wouldn't try to get it (especially since it's a full-blown expansion with extra levels, and not just some purely cosmetic content). Depends on whether the devs want to sell it here, of course.

That would be my preferred way.
I was a backer for Ravensdale and will therefore get Rise of the Owlverlord for free, and having it here on GOG would be great, I would want to wait for that.
(Black Forest seems to be quicker with their Steam releases compared to their DRM-free releases.)