I have always struggled to get my on the run nutrition right. For marathons I found gels are ok but tend to get the sickly after a few hours and for ultras you need solid food.
I had played around with a few gels. First settling on SIS ones, they seemed ok but didn’t react well to other nutrition. I switched to Nectar gels about 18 months ago. They where much better no after affect but occasionally ended up with stomach cramps during a race when just using these and water. Not sure what the cause was as it didn’t happen all the time and if I eat any solid food it seemed to sort it out.