Help with drywall?

I am currently redoing my upstairs of my small home. I have had limited problems drywalling the hallway, but admit I am not a professional. The previous owner installed new framed walls in one of the bedrooms and I have issues here. The roof slopes on both sides and he made the studs go up to the slope in order to have a crawlspace behind them. This is fine except there is a strange gap where the roof meets the new wall. The gap is about 8 inches. What should I do about that gap or should I just drywall and leave the gap behind it? I am so confused.