George answered this question in a recent interview with Meduza on 22nd August 2017.
Q: How independent are the showrunners from you? Simply put: could they save the life of a character you’ve decided to kill? Or
could they kill someone who’s still alive in your books?
GRRM: They are independent. They can do whatever they want. I don’t have any power… any contractual right to [stop them]. I consult
with them. I talk to them on a regular basis. Of course, years ago, we
had a series of very long meetings, where I told them some of the big
twists and turns and huge events that were coming in the last few
books. So they’ve been touching [on] some of these, and doing some of
the reveals, but they have also been departing in various ways.
The biggest one is one that you just mentioned: probably right now,
right as we talk, there are close to 20 characters who are dead on the
show, who are still alive in the books. Some of them are very minor
characters, but also there are major characters, like Rickon Stark,
Barristan Selmy, Myrcella Baratheon. All of them — dead on the show,
but alive in the books.
There are also a number of characters in the books — fairly important
characters — who have never been in the show at all. Characters who
were omitted totally. It’s not a question of killing them; they’re
just not there. They were never a part of it: Lady Stoneheart is one
of them; Arianne Martell, the heir to Dorne, who’s a viewpoint
character; and Victarion Greyjoy, one of the sons of Quellon Greyjoy
and brother to Balon and Euron. All of these characters are quite
important in the books and missing totally in the show.
And regarding his involvement in the show as compared to D&D's, he said the following in a separate interview to Time:
Q: How has your involvement in the show changed over time?
GRRM: I’m a co-executive producer on the show; David and Dan are the showrunners. Right from the first, we knew that they were going
to do the lion’s share of the work, but I did wanted to be involved.
Initially, I was involved in all the casting — I wasn’t physically
present — I was here in Santa Fe. But through the wonders of the
internet, I was able to look at all of the actors reading and to write
them long letters and to have phone calls where I discussed which
actors I like and which actors I didn’t like. And in the early
seasons, I wrote one script per season. I would have gladly done
more, but there just wasn’t time. I’m still trying to do these books.
It takes me about a month to write a script and I didn’t have a month to spare, so I said, I think I’ll sit out season 5. I’ve sat out
seasons 6 and 7 too, just trying to concentrate on this book, which
as you know is massively late. So in that sense, my involvement in
the show has diminished over time, though, I’m still here whenever
they want to talk to me, and I’m always glad to weigh in. David and
Dan have come to Santa Fe and we’ve discussed many of the ultimate
developments, those landmarks that I spoke to at the end of the road
that we’re both driving for. So I don’t need to be quite as involved
as I was at the beginning.