The main problem is that there is hardly any dialogue. They'll have to find some way to put across her thoughts and emotions as she goes through her ordeal. Even in a movie like 'Buried' (2010) which takes place in a coffin there is someone talking on a phone. Gerald's Game doesn't have that. It's been a while since I read it and it's one I've never read again, because I liked it so much I've always been scared that the second time it wouldn't work as strongly because I already knew the major twists. I seem to recall though that she also hears the voices of other people she knew: so probably they'll do those voices as voice-overs. Maybe they expand the backstory and have longer flashbacks to her youth and other periods in her life... I assume that just as Dolores Claiborne they have to expand it somehow.