Gu is a gel that is made up largely of sugar. It comes in two-ounce packets and provide about 100 calories of energy in a form that is quickly available so is perfect for an energy boost when you are running for long distances.
Gu comes in several flavors, including orange, berry, chocolate, vanilla and plain. They also have flavors that contain caffeine, which gives you an added energy boost. I can only stomach the plain, as the other flavors seem too sweet to ingest while running. The packet is small enough to fit in the key pocket of your running shorts. When I take more than one, I simply pin them to my shorts and tear through the top when I want to eat one. The top tab is pretty easy to remove, even when you are still running and your hands are sweaty.
After ingesting a packet of Gu, you will notice a little burst of energy after about 10 minutes. It is suggested that you ingest one every 30 minutes, but I find that it makes me nauseous if I take them more often than one every 45 minutes or so. You also should take them with a lot of water, not doing so can upset your stomach.
In the course of training and running half a dozen marathons, I've ingested gallons of Gu, one two-ounce packet at a time. I have tried other sports gels, but I always go back to Gu. They seem to work best for me, and I highly recommend taking a few of these along next time you have to be physically active for more than a hour or two and won't be in a position to eat something.