Spoilers if you haven't finished the series
I've recently finished this book and started look around the net to see what people say about it. I've noticed that one that things that comes up is this book goes against what happens in comic. I've not read the comic, I can't stand the style of the art, that said the way I think of it is that the events of this book are a different loop then the comics. With each loop things get better, so perhaps in this loop Gilead managed to hold out longer. There is some logic to this as well as this book was written and released after the dark tower and at the end of that book he started out with the horn.