This is one of them convenient myths
GRRM didn't just give the ending he gave key major plot points. This gets glossed over because nikkas wouldn't like some of the decisions GRRM made and prefer to just blame them on D&D
D&D didn't make anything up, they just botched what was in front of them, especially with no concrete books to draw from
Will Game of Thrones on HBO end the same way as George RR Martin's books? The author himself has something to say on the issue.
www.forbes.com
"This is a question frequently asked of Martin, and now even more so as we are in the final eight days of the series. And fundamentally it seems that yes, Martin has indeed shared at least large portions of his ending with showrunners so they can execute his vision of the conclusion of the series if they so choose. Here’s what he said
in a recent interview with Rolling Stone when asked if he would be “surprised” by the ending:
"Well, to a degree. I mean, I think … the major points of the ending will be things that I told them, you know, five or six years ago," Martin said. "But there may also be changes, and there’ll be a lot added."
He gave them the major details about the ending, not details of how the story plays out. D&D decided to rush the ending because they were tired of the show and it resulted in a drop in the quality of the writing.