The gullet has a metal bar in it that comes out, leaving only the more flexible frame. They sell gullets in a variety of widths. A narrower or wider one fits in there and the saddle conforms to it. It's not something you'd want to switch back and forth, but something to do when you buy it to ensure it has the best fit for the horse it's going on.
Now there was a nifty saddle I saw at the shop where the gullet inside actually spreads or narrows with an allen wrench. You could do that on the fly, if you were swapping horses.
Cool! Sounds really handy. I'd like to see that.