Do you always write chapter by chapter when you draft? Or do you ever end up with gaps in the initial draft that you have to go back and fill?

No, I don’t write chapter by chapter. I generally start at what I think is the beginning and aim for what I think is the end, but those are guidelines, not rules. I always wind up with holes. Going back and figuring out what belongs in the holes is fun. The trick is to play out one of the story threads naturally, not to cram in a scene just so have something in Chapter 7. If it doesn’t fit, throw it out.