spl - is this not an organized century? If so, rely on the food that is provided. It is far better to eat real carbs such as fruit and peanut butter and bread than power bars. And eat lunch - real food. Just do it. Save your Gels until the last 20 miles or when you start getting fatigued. Then i would say gel every 20 - 30 minutes or so to keep your energy going.
I have just found that my body responds better to real food and not "manufactured" especially on long ride and after 4 - 5 hours in the saddle. Not certain your speed or fitness level but most people do a first century in 8 - 10 hours. Do you really want to eat power bars for 10 hours??? Also don't weigh yourself down with too much stuff. One reason to do an organized ride is so you can travel light.