I got in a very public fight on Facebook last year with my sister. She thinks that GMOs should be labeled, and I think that's not a good idea.
Her ideas make a lot of sense on their face. Shouldn't customers know what's in their food? Let people make their own decisions about what they want to consume! I understand that position, and it's very appealing. It's when you dig deeper that the need for these regulations starts to seem less clear.
Labeling something "Product may contain genetically modified ingredients" is more or less fear-mongering. We don't label packages with everything in there: "Product produced using dihydrogen monoxide" or "Ultraviolet rays of stellar origin used to propagate this product." Labels by themselves are an implied warning: they make people wary, which in this case is unjustified.
In fact, some GMOs are good for the environment: if fields are made more productive or pest-resistant, they reduce the incentive for agriculture to expand into wild areas. (Other GMOs, such as glyphosate-resistant crops, basically encourage farmers to use lots of herbicides.)
Second, labeling is expensive. If every ingredient has to be carefully tracked, that takes work, and work costs money. These labels are sure to raise the price of food.
There is a longer discussion here, including points in favor of and against labeling GMOs. It's more complicated than it sounds! Take a look.