First of all, if you've been eating regularly in the days leading up to the century, you've got about 2000 calories in storage in your liver and in your muscle cells (but this is different from what people talk about when they talk about burning muscle).
If you burn about 500-600 calories per hour on the bicycle, you could go 3-4 hours without eating anything, and people who are experienced century riders can do this.
Second, if you have a good breakfast of 500+ calories, you'll have an additional 500+ calories available to you, which should give you another hour.
And there are experienced century riders who eat a decent breakfast, and then head out and ride a whole century on a couple bottles of sports drink and a granola bar. You do actually have enough in storage to do that sort of thing, but it seems to take some experience with riding long distances to be able to do it.
That's why the recommendation is to consume about 250 calories per hour. You consume 250 calories, and your body accesses another 250-350 calories from what you've got in storage each hour. You can last a long time without bonking that way.
They've also discovered that the stomach has trouble digesting much more than 250-300 calories at a time while you're exercising.
As for food ... keep the gels for emergency use. Look for small tasty food that is going to provide you with a lot of calories. Oatmeal raisin cookies, for example, can run 250-300 calories. One of them per hour would be a good choice. I like to supplement my cookies with things like beef jerky, salted almonds, potato chips, oatmeal chocolate chip cookies, pastries, etc. etc. ... whatever grabs my fancy at the time.
And whatever you use as a sports drink can be counted in the calories too. I use HEED on the bicycle, but when I stop at a convenience store, I go for either orange pop or 100% pure orange juice. The orange pop gives me somewhere in the neighborhood of 200 calories if I recall correctly, plus it provides me with an extra 550 ml of liquid.