GRRM is a co-executive producer actually, so he's not there. He's rarely there, actually. He writes his single episode per season but has no official say over the direction of the show (as evidenced by things that have been done that he is not fond of).
I didn't mean literally there, like sitting there while they work.
I meant he's involved, and has said on numerous occasions he's told the producers which specific directions he's going. And he pens an episode per season, so he's directly involved. It's foolish to think D&D don't know virtually everything about Winter, and have a real good idea what the bullet points are for the last book.
Winter exists in some form already, and is fully 1/2 of the remaining story.
My overall point is that for a book-to-TV production, the producers have stayed the course almost exactly and that seems to be due to their explicit intention to stick very closely with book plot, and the fact the author is directly involved with the show on various levels.
Deviations have been very minor overall. Almost all storiy arcs end in the same place they do in the books, and where they don't, generally they are minor plot points. (see the Ironborn's fear of dogs, etc.)
