Lots of good advice (as always
) for what it's worth, I think experimenting on what's right for you is the way to go, for me strangely I never use gels in training runs, I typically run between 15-18 milers 3/4 times a week and get through fine (for me) without, no water either (unless I pass a fountain on route) but that's because I hate to carry anything, tried it but found it awkard.
However on race day i'll probably use 2 gels during (GU brand nice flavours) when I remember to pack them! But I don't know how much they actually help? My PB in Rotterdam was run with no gels (I forgot them!) just the Isotonic drinks and water on the course and I had a great easy feeling race, where as other marathons where i've used them I haven't found it any easier as I would have expected.
For me I think gels are as much a mental boost as physical in my opinion based on my performance, Jelly babies/beans are much nicer and cheaper but not as portable.
One thing to add not taking anything away from people who use gels, but the human body does adapt, which is evident through progression in training.
Anyway best of luck in training and your planned event.