Energy requirements vary between individuals, increasing with intensity. The general rule of thumb is to feed yourself with one gram of carbohydrate per kilo of body weight, each hour – so a 60 kilogram individual should aim for 60 grams an hour, while an 80kg rider would need around 80g.
Consuming too much carbohydrate means it'll be surplus and go to waste – however, some manufacturers mix their glucose with fructose (fruit sugar) – and research has shown that this can increase absorption to 90g an hour.
Some gels come with additives such as caffeine. Studies have shown that caffeine can boost performance when administered in the correct dosage - but the milligrams per average cup of coffee can vary wildly. The amount in a gel will always be uniform and thus dosage can be properly tailored to requirement.
There's one other really important thing to remember: make sure you take your empty packets home. Cycling Weekly is fed up with seeing discarded used gels out on the lanes, even if they accidentally fell out. Let's leave no trace when we go out riding.