You don't need glucose packets. They might help speed you up a bit, but as long as you've built up your range progressively, the human body can go without them. As I mentioned above, I used to do > marathon distances once or twice a month in the mountains for fun (which I realize is very different than racing. I did dozens if not hundreds of 50k+ runs and have never yet even tried a gel pack. Some of those runs were at a weight of over 90kg!
Yes, you'll burn a ton of calories. I often found I weighed in at a full 5kg less after a run and probably 0.5kg less after re-hydrating. Sure, I was never anywhere near as good at long distances as mid distances, but you don't need gel packs. At least I didn't for anything up to 50k, and I probably could have gone further if not for the logistics of carrying enough water and dealing with the heat and humidity that go along with living on a semi-tropical island.
I used to puke anything I had when I ran.
My digestion couldn't handle food when I was running. It was a mix of slight heartburns (& coffee) causing a sensitive stomach which didn't like food moving around when I ran and partly because I was running so hard my stomach didn't have enough blood to process the food.
Several weeks of eating peanut butter mini sandwiches with their crusts cut off, banana slices and oranges dipped in salt, while running at a moderate pace, I managed to tame the tummy.
That said - I don't think I like the taste of GU gels, but like coffee or beer, you get used to it (& the extra 100Kcal feels like it goes directly to your legs).
Try something like flapjacks or even rice cake [1]. It's more suitable as bike nutrition because of the ergonomics but at the end of the day it's all just carbs.
If you don't like any of that or the solid food is difficult to swallow in a race, you can try just water mixed with maltodextrin. It's a very slight sweet taste.
1: https://www.britishcycling.org.uk/zuvvi/media/bc_files/izone...
Personally, I've switched to mostly liquid nutrition, currently using a custom blend from Infinit. For 6+ hour races, I'll supplment with a PBJ sandwich or a bar of some sort.
And like another person mentioned, it takes some practice to get used to it. A lot of people have trouble digesting anything while running.
I found that just like my legs needed training to withstand a marathon, so did my digestive system need practice to process and retain enough calories. Eating 4000+ calories on a long run day used to be pure torture. But you get used to it.
I can run a 13mi run, and maybe some extra, without gels. Even without water. But in my experience I can't do it for a 20+ mile run.
And my fasted runs are generally slower than nonfasted runs regardless of how I actually feel while running it. For example, when I train every day, my morning runs before a full day of eating are abysmal. My evening runs are nice and fast.
Worse still, when training every day, I've noticed that what and how much I eat the evening before, has a huge impact on my morning run as well. (I tend to do my long runs early in the morning because fitting a 4h run in the evening is tough)
Maybe my problem is the every day training part. It makes nutrition crucial so I can get enough recovery out of my body.
Low-ish carb is great. It helps you endure long runs overall and it's a good habit in general. Especially for recovering after the race and getting all those calories back.
During the long run itself however, glucose packets are absolutely necessary. You run out of glycogen stores every 45 minutes or so. That's when you start cramping up and having trouble. A glucose packet gives you some caffeine (helps with stamina and pain tolerance), and some electrolytes (helps with cramping and hydration), and some 100 calories of sugar (helps with glycogen levels and mental fortitude).
Crucially, you aren't replacing all the calories you burn. That's impossible. In a 4 hour marathon I burn between 2500 and 3000 calories. The packets replace only 500 calories.
The difference between packets and no packets is that with packets, I am able to be a human person after a marathon. Far less cranky, far less tired, generally feeling okay. Without the packets, I'm basically a walking zombie.
That said, it took a lot of practice to figure out the right timing for those packets, to train my body to absorb them and so on. It's definitely not something you want to start experimenting with on race day. That's a recipe for puking.