Growing your plants in a greenhouse to avoid pollen contamination seems a bit over the top and as noted corn is not a plant that does well in greenhouses (it is wind pollinated so you would have to have serious fans moving the air). And they are big plants. To have enough room to grow any significant amount of corn, you would have to have a very large, expensive greenhouse.
Because when we eat corn we are eating the seeds of the plant, it can be affected by cross pollination. (Squash for example we don't typically eat the seeds. So if it gets cross-pollinated by another variety, only the seeds are affected, which only makes a difference if you save those seeds and plant them the next year). So yes, if your corn was exposed to the GMO pollen at just the right time when it is in silk, it could become GMO corn.
The ways to prevent this include the 600 ft spacing, having a windbreak between your property and the GMO corn fields, growing a different variety that matures at a different time, so it is not in silk when the GMO corn is pollinating, or changing the planting time to do the same thing.
I agree that the GMO corn, even if yours did get cross pollinated is not dangerous. As imafan noted the point of the modification is to make the corn resistant to RoundUp, so they can spray the fields with RoundUp to keep weeds down, without killing the corn. So the RoundUp is the problem, not the corn. I am not so sanguine as imafan about the safety of all that RoundUp, for us and definitely not for the environment. This thread https://www.helpfulgardener.com/forum/vi ... 11&t=57653
has a lot of the information I collected on effects of RoundUp and other poisons. So what you should be concerned about is not whether some of the GMO pollen gets on your corn. Even if it does and even if you eat it, it will make no difference and have NO effect on you. What you should worry about more is whether the RoundUp they are spraying drifts onto your property.
Whether or not you put poisons on your food does not change their nutritional make up. What does change the nutritional make up of the crops is the amount of nutrients in the soil, it's richness.
https://www.ivlproducts.com/Health-Libra ... Nutrients/
Studies show that crops grown in the past were actually healthier than crops grown today. Modern agricultural methods have depleted nutrients from the soil which results in less nutritious fruits, vegetables and grains.
Researchers from the University of Texas in Austin examined data from 1950 and 1999 for 43 varieties of fruits and vegetables. They found "reliable declines" in amounts of calcium, iron, phosphorus, protein, riboflavin and vitamin C over the nearly 50-year time period. The team attributed the results to modern agricultural methods intended to enhance crop size, rate of growth and resistance to pests and disease.
According to the Organic Consumers' Association, many other studies report similar results. An analysis by the Kushi Institute looked at nutritional data in 12 fresh vegetables from 1975 to 1997 and found that calcium levels were reduced by 27%, and iron was lowered by 37%. In addition, levels of vitamin A fell by 21% and vitamin C was reduced by 30%.
Another study found that the amount of vitamin A our grandparents would have gotten from one orange equals the amount found in eight oranges today!
If you want to get deeper into the subject, this https://grist.org/article/2009-08-13-deb ... nutrition/
is a very nice article, a debate between two reputable scientists about whether/ how much organic crops are healthier than chemically raised ones. Again the debate centers not on the poisons, but on the soil quality.
In general, organic farming focuses a lot more on building the quality of the soil. But that is something I can do much more intensively in my 500 sq ft of veggie gardens than a farmer with hundreds of acres can, even if they are organic growers.