The gluten free diet is a clear one for all us Celiacs, but the grain free one, not as much.
However, we're all competent adults, and usually ones who pay attention to our health because we've had to do so. It seems to me that some people do just fine with grains, some people are knocked flat by 'em, and they are typically aware enough of their own bodies to tell the difference, you know?
Whether it's the grains themselves, or GMO issues, or pesticides, or whatever, if it works for some people to avoid it, more power to 'em. God knows I've struggled enough to figure out my ridiculous body that I'm always happy to support anyone who's found the answer to their own health issues.
As for myself, if I eat grains, I'm sick. My kids' health issues are much better when they have a low-to-no grain diet. Previously, when eating grains, I had vitamin deficiencies and other medical problems that wouldn't resolve until I made some major dietary changes, including going grain free.