Gravity has nothing to do with how spiral galaxies work, that’s the whole point. In fact gravity could not even begin to hold galaxies together given the distances and sizes involved. Other kinds of things which we read about in the media, such as black holes, also turn out to be fiction.
Ok, so what does hold galaxies together? Why do the spiral arms rotate around the center? Why don’t they just fly apart? Why do they look just like a hurricane?
Must be some kind of force not yet determined or explained that I know of. I’ve never heard of a concrete explanation, but then again, I’m just a layman.