This is not a matter of not understanding nutrition, but knowing that much of the population is suffering from vitamin deficiencies, whether they are gluten-free or not. Being gluten-free simply means that you lack access to the most commonly fortified food - wheat flour - and many gluten-free flours are not fortified at all. I'm fully aware that wheat, barley, and rye don't contain any vital nutrients that can't be found elsewhere, but I am also aware that there is a reason wheat has been fortified for more than 50 years despite that there aren't any regulations requiring it.
Edit: The 2002 study out of Sweden that found vitamin deficiencies in people who had been gluten free for 8-12 years.
I would tend to disagree that much of the population suffers from vitamin deficiencies. That is alarmist stuff. Those with malabsorption issues would have to worry about it but if they learn about nutrition and where to get the vitamins and minerals needed, plus address their malabsorption problem, it should rectify itself. If you eat a balanced diet and take appropriate supplements, most people should be good to go. It really is all about learning how to eat. Many people today have no clue what's in the food they eat and tend to gravitate towards low quality carbs. I am sure there are people who are vitamin deficient but I doubt its "much of the population". That is like saying most people should not eat gluten. I don't buy into the idea that everyone has a problem with gluten, either.