In my opinion, someone great to all of time is someone who brings about change. George RR Martin has brought about that type of change. The most obvious way is how he writes his female characters. A lot of action genres present women who are cardboard cut outs made to serve the back up "hot" female role. However, his female characters have depth and development and serve just as much purpose as his male characters. And this can help bring about a great change in our media representation of females in the media.
Yes, George RR Martin is one of the greatest writers of all time, because one of the ways a writer's greatness is measured is the writer's fame. Martin connected with readers. He had many fans that loved his work during his lifetime. For this reason alone, Martin can be considered one of the greatest writers of all time. His writing really speaks to people.
Martin is just like any other hack fantasy writer, he just had the foresight to realize that including tons of violence and boobs is a surefire way to get his books adapted into an HBO show. Even more insulting is that people claim he's progressive. Every woman in the Song of Ice and Fire novels are sexualized objects, and that includes women under 18.
George RR Martin is not one of the greatest writers of all time. I have never heard of this person. I consider great writers to be those commonly studied in school, such as Shakespeare, Shelley, and Faulkner. I never studied Martin, nor have I ever heard of him, thus he is not one of the greatest writers of all time.