I only run half marathons. Don't run anything longer distance than that but I found that gels gave me a very upset stomach. Won't go into detail.
Got some advice from a thread on here a while back.That was, to try dextrose tablets. I buy "Dextro Energy". A pack of 14 tablets cost about 60p from most supermarkets. I have two tablets about 10/15 minutes before a race or long run and another 2 an hour into the run. They seem to help me with energy levels. I also used to get quite nauseous after long runs too and that's stopped.