I've never had any reason thus far to use Gu or the like, but for day-long fencing tournaments, I'd bring plenty of water (to go with what they provided), pretzels (carbs+salt), nuts (protein) and fruit of some description (usually apples or bananas). Obviously what you're going to be carrying around on a bike is going to be a little different from what you can carry in a gym bag, but something along those lines works if you want real food.

(I'm pretty sure that I'm going to try for real food when riding...the texture of some of those things--yuck!)