Thing is its a very mature TV series which means most of its viewership are going to be older. By that stage in life chances are they've read more than one fantasy book in their life up to that point. So for those who are more likely for Game of Thrones to spark a fantasy interest; if asked about their first reading experience/gateway (many will mix the two up), they won't think of Game of Thrones.
So its actually where I'd expect it. I'm sure its inspired many, but its unlikely to be many peoples "first". That's why we see so much more impact from Harry Potter, Tolkien and Narnia. One is a kids book the other two also have kids books, but are also very firmly on all those lists that advise parents what their children should be exposed too (either reading too them or having the child read the book). I fully expect that Harry Potter will make its way (and likely already is) onto those lists. Plus with the media hype, films and franchising I expect it to join Tolkien and Narnia as long term addition. It might possibly get pushed out by a new generation young book.