Don't forget substitution effects. As it turns out, grains are pretty fungible. (Yes, it's nice to be able to use that word in its normal context for once.) As wheat becomes too expensive, demand for others will increase and their prices will spike as well. So yes, gluten intolerant folks will be significantly affected as well.

I think shortage of grain makes for a global food shortage, which causes tons of problems.

First off, the people that mainly will be affected are the poor in poor countries. Most in developed countries will probably be fine.

That said, the rise in these grains will likely spill over to other foods, as people turn to substitutes for their calories.

If there’s a wheat shortage, people will be eating more rice than usual.