In short, Stephen King is one guy who doesn’t mind killing lots of trees in the name of a good yarn.
At least a novel filled with characters presents multiple opportunities for interaction. In Gerald’s Game, Jessie is mostly alone, except when she’s being menaced by The Space Cowboy.
Name a prototypical King story and one’s mind immediately goes to something like IT, The Stand, or even his multi-novel epic The Dark Tower.
In some ways, adapting Gerald’s Game might actually be tougher than taking on one of King’s sprawling novels.