Yeah, I find the gels give me about 250 additional Watts....
Wait.
No. That's the EPO that does that.
The gels are good for some calories and sugar energy...caffeine, too, if you get GU Roctanes or similar. They work from anywhere between 10 and 20 miles, depending on tempo and energy state of the rider (just my down & dirty estimate). I might get 25-30 miles into a ride and start feeling the need for more power and the gels seem to get my energy level back up...usually.
Three or four gels is my max. They tend to knock out my stomach if I over-do them. But, like yesterday's ride...it's just clip in...keep eating as you roll...boom!...back at the car without missing a beat. Two gels, two bottles, a few hard candies...no hunger knock. And nothing ingratiates yourself to a training partner as handing him a Tri-Berry gel just as he's detonated with 10-15 miles left to get back to the cars.