This seems to be a fair and reasonable request to know how our food was created. Food is already labeled with its ingredients when it has been processed, and warnings are sometimes included on labels. So, why not let the consumer know if any of the components of the food were GMOs?
The reason food is not labeled as containing GMOs is that mandatory food labeling is only used to provide information that may be important for consumers to make food choices regarding ingredients known to affect their health. After many studies and years of experience with consumption of GMOs there is no credible evidence that there is a health risk associated with eating GMOs.
The most consistent association between health and consumption of GMO food is a beneficial one that comes from GMO maize with the BT protein having significantly reduced levels of mycotoxins. Mycotoxins are naturally occurring toxins produced by fungi that are known to cause serious health problems when consumed. GMO maize modified to contain BT protein has much lower amounts of mycotoxins than do non-GMO maize plants since the BT protein reduces insect wounding of maize; the wounds provide entry points into plants for invasion of the fungi that produce the mycotoxins.
GMOs are created by taking specific genes from one organism and moving them into another. Most often a naturally occurring gene transfer system is used under carefully controlled conditions to move a desirable gene into a new organism. We know exactly what is transferred and where it is within the genome of the new GMO. This is in contrast to traditional breeding techniques where genes of two different individuals are mixed together and result in a cacophony of new individuals that like a GMO are new to nature. A major difference between the two methods used to create these new individuals is that we know much less about the genetics of the organism created using traditional breeding methods than we do about the one created using modern gene transfer methods. The mixing of gene variants by breeding can result in major changes, as we have learned from the breeding of dogs. We have benefited enormously from traditional breeding methods to create the diversity of foods on our tables. But, we can never predict how mixing these gene variants together may affect our health when eaten, something that is much easier to predict for the individuals created using modern gene transfer methods. Furthermore, new GMOs are specifically tested for their effect on our health, while no such testing of new individuals created by traditional breeding methods is done.
Is it then fair and reasonable to require mandatory labeling to warn consumers that food contains GMOs when this labeling system is only used when health risk choices must be made by consumers? Without evidence that GMOs are a health risk we should not compromise the integrity or credibility of our food labeling system by requiring a warning when there is no credible scientific evidence for adverse health effects being associated with the consumption of GMOs.
Foods can be and are labeled to help consumers make choices, but such labeling is voluntary. Common examples are kosher and halal labels that help consumers select or avoid foods based on their belief systems.