I know a lot of folks have strong feelings about genetically engineered foods, but regardless of whether you think GMO foods are evil or are perfectly safe, there are plenty of good reasons to vote NO on Prop 37.
First, I begin with a presumption that making law by initiative is generally a bad idea. The initiative was created as a failsafe mechanism for the electorate to guard against a corrupted legislature, and should only be used when absolutely necessary. It is a very poor way to make legislation in general, particularly legislation that is complex and gets down to the details of regulations, such as what exactly has to be labeled, what the labels have to say, and what the exceptions are. Detailed legislation like that is often imperfect and later needs to be corrected. The State Legislature can amend the laws that it passes, but initiative statutes can only be amended by another initiative. Even in the "pro" and "con" arguments for this proposition, both sides point to details and exceptions. This is the sort of legislation that is likely to be flawed and to want amending, making it a bad candidate for an initiative statute.
Second, this sort of food labeling regulation is not something that should be undertaken at the state level. We currently have federal regulations from the USDA defining similar issues such as "organic" labeling and "milk from cows not treated with rBST/rBGH". The federal regulations concerning milk provide a framework giving some latitude for variation within states, which is viable for milk because milk is generally an intrastate business. For produce and even more so for commodity crops like corn and soybeans, which become ingredients in many processed foods, the interstate and international commerce is substantial. Imposing a California-only regulation on this interstate and international supply chain would create a complication for all California food producers and for global food producers who wish to sell in California, which ultimately means added cost for the consumer. It only gets worse if other states decide to get into the act. That is exactly why such things need to be consistently regulated at the federal level, as has been successfully and effectively done for organic food and growth hormone-treated dairy cows. Moreover, it is probable that the USDA will eventually provide regulatory guidelines for non-GMO foods that will preempt California's initiative, meaning that everyone in the agricultural supply chain from farmer to grocer will have been jerked around by California's initiative.
Finally, the approach proposed in Prop 37 fails to follow the successful and effective path already established by USDA "organic" and milk labeling regulations. In the case of foods labeled "organic", the USDA regulations standardize the criteria that must be met in order for a food to be labeled "organic". Note that this is a voluntary positive claim. Food producers who believe that consumers will find value in meeting "organic" standards can choose to meet those standards and put the label "organic" on their product. Products that do not meet the standards are prohibited from claiming to be "organic", but they are not required to put any adverse "non-organic" label. Under this voluntary "make the claim only if you can meet the standards" regulation, the organic food industry has thrived. As of 2010, U.S. sales of organic food and beverages were a $26.7 billion business, and represented over 11% of all fruit and vegetable sales. This approach has given the consumer good choices in the marketplace, and the ability to make their own choices about the value of "organic" food, which tends to command a higher price. A similar approach was taken with the growth hormone issue in dairy cows, where dairies were allowed to market under a standardized label stating "milk from cows not treated with rBST/rBGH" if they met the criteria, but were not required to put any adverse "contains rBST/rBGH" if they didn't meet the criteria. Here again, this approach enabled choice for the consumer and the development of a substantial niche market for those willing to pay a higher price for milk from untreated cows. Rather than taking this proven pro-consumer and pro-market approach, Prop 37 imposes a negative involuntary regime where all food producers who do not meet the criteria are required to put an adverse "contains GMO" label on their products. Unlike the voluntary USDA approach that allows food producers to decide if and when they wish to make a claim that some consumers may find valuable, the Prop 37 approach imposes its regime on all producers, which is likely to trigger supply chain disruption and higher prices as food producers scramble to meet the new California-specific requirements. (Not unlike California's specific requirements on gasoline, and we've seen lately how that can disrupt a supply chain and affect prices.) When we've already seen how well a voluntary claim system can work, it would be foolish to take Prop 37's negative involuntary approach risking adverse affects on farmers, grocers, and consumers alike.
Thus, regardless of how you feel about GMO foods, I hope you will agree that Prop 37 is a bad idea.