The basic idea of an energy gel is to help maintain your blood sugar levels without you having to stop to eat something. It has to be easy to swallow and to digest. Our body absorbs simple carbohydrates faster than complex ones. If a particular gel has different types of carbohydrates (like glucose, maltodextrin, fructose, etc.), you'll absorb them even faster because there are different receptors that can absorb them at the same time.
Where it gets complicated is when you start to think of what other ingredients you might want in your gel to help with performance, recovery, etc. A bit of sodium is definitely desirable for long races or if you're sweating a lot, to prevent cramps and other symptoms of electrolyte imbalance. Some gels also have vitamin C, vitamin B complex, and others. I'm not sure if they help with performance, but they probably help with recovery.
That's basically it. Other more expensive gels might have additional ingredients like caffeine, taurine, amino acids, etc., but they're not as important. "All natural" is just a silly sales pitch that doesn't have any scientific basis.
For a 100 km race, most people will alternate between running and walking. In this case, I think it would be better to just eat a snack while walking. For really long races you'll probably need other nutrients as well, not just carbs. I'd recommend eating real food whenever you can and supplementing only when necessary. Expensive energy gels generally offer very little additional benefit, so I personally prefer the cheaper ones.