Before you do anything that involves pain or restraint, work out a signal that she can use if she wants to stop. You must always remember to stop when she asks or you will break her trust in you. As for leaving, don't go very far because if something bad were to happen (she chokes on the gag, the house catches on fire) you'd never forgive yourself.
As for the Great Gatsby, that's a bit much really...besides, it will make her feel worthless if you tie her up, gag her, and then go read a book. She's offering to let you try something sexual with her. If you sit there and read a book instead, she will think you're not sexually attracted to her anymore and it could hurt your physical relationship with her.
Couldn't you read something better to her after you have sex with her? Hell, rent The Great Gatsby if you must (personally, I'd choose a different title but that's just me) but read it to her? If she's not well-read then she won't understand the literary value of it anyway. Why not suggest a book club instead?