Simply put,
feeding your insects is whatever you do to keep them alive, possibly until they're old enough to breed. So the food you give your insects can really be anything that they will eat that will be nutritious to them.
But
gutloading your insects is the process of filling their stomachs with nutrients that they will pass directly on to your chameleon when it eats the insects. So gutloading needs to take place directly before you feed the insect to the chameleon (usually anywhere from 48-12 hours before you feed off the insect). Think of it as the insect's 'last supper'...
Whatever is undigested in the insect's stomach when it is eaten will be absorbed by your chameleon. That's why there are such specific gutload recipes. The nutrients in the gutload aren't necessarily required to keep the insect alive, but they do hold nutritional value to your cham.
You can't get a cham to eat alfalfa or brewer's yeast (etc.) directly, but if that ingredient is inside the insect's stomach when the chameleon eats it, you are able to pass those nutrients on to the cham.
You could easily keep your crickets/roaches/etc alive for many weeks by feeding them only cheap 'foods', provided you feed them quality gutload in the last 12-24 hours before you feed them to your cham.
But the gutload is only good for your cham whilst it is undigested inside the insect. As soon as the insect has absorbed nutrients from the gutload and excreted the rest as waste, it cannot be considered a 'gutloaded' insect any more. So if you leave uneaten crickets roaming around your cham's cage for more than a day, they will lose the nutritional value they had when they were gutloaded. That's why it's important to take uneaten ones out of the enclosure and back to their bin for more gutloading.
Read through the following sites for more information regarding gutloading:
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Chameleons! Online E-Zine: Gutloading Basics
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Gut loading feeder insects
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Gut-loading feeder insects
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Successful Keeping of Veiled Chameleons: Feeders
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The Chameleon's Dish: Your Nutrition Plan