Gels in general and the first one I used was at the end of the 100 mile offroad we used to do. The final 3 hills are killers and it may have been the tiredness- but it seemed that they save the longest-steepest ones till last. Got to the 2nd to last hill and it nearly finished us. Took a gel at the top and plenty of water and 15 minutes later hit the last hill. Won't say we sailed up it but it was not that bad. But the next 3 miles across the golf course were done in the big ring at 25mph and I can't even manage that on a normal ride.
Only problem I have found with gels is that some do not taste very good. None of them do but some do taste a lot better than others. You have to find the ones that work for you- and you can keep down when you are shattered.
__________________
How long was I in the army? Five foot seven.
Spike Milligan