One minor point may have been missed in the above answers. High-intensity exercise burns a lot of energy, and you mainly get that from carbohydrates. Thus, you definitely should use gels on fast rides. You can use them on long slow rides, or you can eat solid food, or you can mix the two - whatever your stomach can tolerate. For hard interval sessions, if you do them, then even for hour-long sessions, gels can help - although you can just take a couple of teaspoons of something like maple syrup. I'm serious about this, I have a Costco jar of maple syrup next to my trainer for this.
The conceptual diagram below shows the relationship between exercise intensity and the proportion of energy derived from carbohydrates. It's taken from a discussion on Road Bike Rider.com, and it originates from an academic article.
The y-axis is calories per hour for a reference rider of unspecified weight. Being an older article, the x-axis is exercise intensity, parameterized in terms of percent of max heart rate - low intensity is 65%, moderate is 75%, high is 90%. In modern terms, I think high is probably around functional threshold or higher; for the OP's reference, functional threshold power is the maximum you can sustain for about an hour, and it's not that comfortable to hold but it is doable. Regardless, the point is that even at moderate paces, you burn a lot of carbohydrates. As discussed, these come from muscle glycogen and blood sugar. If you deplete that, you will bonk (also called hitting the wall).
Relatively simple carbohydrates like energy gels will, as discussed, hit your bloodstream faster than if you eat real food, and they take up less space in your stomach. For a race of an hour or so or a similar interval session, you could take a gel before the start. Although not everyone feels they need to do this, there's some research showing that this is beneficial despite the short duration. For rides that are both high-intensity and long, I find that gels or energy chews are very helpful. Some more modern recommendations are to consume as much as 60g of carbs per hour in these situations, and this does require some adaptation (your stomach may revolt if you eat that many gels without prior exposure). For rides that are long but low-intensity, I think you should experiment to find what works, but definitely consider something like gels.
If you eat natural foods or non-specialized prepared foods like cookies, granola bars, bananas, the fat, fiber, and protein content will slow the rate that carbohydrates are absorbed. You can consider this in your fueling plan.
One minor note is that professional cyclists may have high rates of oral disease. They ride very fast and very far, and they take in inhuman quantities of sugary energy drinks and gels. For us mortals, I would merely counsel taking a sip of plain water and rinsing your mouth after an energy gel or drink, just to reduce the probability of suffering tooth decay. I think there is some evidence that some people are more prone to dental cavities than others. If you have a history of cavities, you may wish to consult a dentist, but be aware that their recommendations might be very conservative if they're not also an athlete.