Sorry to ask a really stupid question but what happens to you gluten intolerant folk if you eat gluten?
It depends why you are gluten free.
Some people go gluten free because "wheat bloats them" or for other reasons that have nothing to do with allergies. For these people not much happens as they are choosing to be gluten free. Mostly they may just feel a bit bloated.
Some go gluten free because it triggers IBS symptoms. These range from mild stomach pangs and slight d&v through to I feel like my insides are being turned inside out stomach cramps and d&v so bad you spent hours on end in the bathroom and wish you had a hasmat suit.
Others are coeliac which is an autoimmune disease. For these people not only do they get the nurovirus style epic cramps and d&v but they also get the pleasure of their body utterly breaking as the gluten literally attacks their body.
For both coeliacs and ibs sufferers with a severe intolerance symptoms can in some cases last for weeks or even months. It's why avoiding cross contamination is so vital.
I once had a pizza which was gluten free but they must have had lax handling practices as both myself and my coeliac friend were very sick afterwards. I had nurovirus type symptoms for 3 days, she had them for 3 weeks.
I have major trust issues and rarely eat out. Too many places think that "it's a fad" and cross contamination doesn't matter.