I think taking gels etc on long runs is about topping up the tank rather than giving you a boost. In other words, it's about delaying the time it takes to run out of energy rather than give you higher energy (adding more fuel to a car doesn't make it go faster or run easier...it just stops it running out of petrol).
I struggle to get anything down on long runs of more than 12 miles. I understand it's quite common because the blood in our body is being directed away from the gut towards the muscles as they need it most.