Update 05/21/2019: George R. R. Martin has again addressed how his final two books in the Song of Ice and Fire saga will differ from the ending of Game of Thrones.
In a post saying goodbye to the HBO show, Martin discussed his long-awaited conclusion to the written version of the series, and addressed the biggest question first: will the ending be the same? His answer is non-committal: “Well… yes. And no. And yes. And no. And yes. And no. And yes.”
His point is to say that, while he’s working towards a similar end goal to the makers of the show, they had six hours of TV to work with and he’ll likely have 3,000 pages of manuscript (and will add more if necessary). The media involved allow for different kinds of expression and explanation.
Source: IGN.com Update: George R.R. Martin on How the Game of Thrones Books' Ending Will Compare to the Show