This game is a pleasant tangent from the 3D Puzzle Room mobile games I used to play.
The new kind of environment interaction and objectives to complete are a welcome deviation from that gameplay formula.
Your GDD is also well laid out with excellent visual aids and references to help bring the vision of the game together. It was concise and posed enough thoughts about each of your subjects (Inhabitant AI, Spooky's tap controls, the Scare-o-meter, etc.) to introduce the idea to potential teammates.
The only thing I didn't either see immediately or understand was if there was a losing condition in the game and what that might entail. You did mention that if the player didn't do any possession or create enough of a disturbance for the Inhabitant to notice and be spooked that their scare-o-meter would slowly drain over time. This doesn't matter as much if the game is meant to be a penalty-less casual experience but if it's meant to be that way it should be mentioned in the document.
Overall this was a fun and colorful read! I wish you and your project here well in your future pursuits!