You may be able claim the difference of the cost between gluten-free food and food that is not gluten free as an itemized medical expense.
IRS Publication 502 states that you can claim the extra cost from special food as a deduction if the following applies:
The food doesn't satisfy normal nutritional needs,
The food alleviates or treats an illness, and
The need for the food is substantiated by a physician.
The amount you can include in medical expenses is limited to the amount by which the cost of the special food exceeds the cost of a normal diet.
The Celiac Disease Foundation has provided a good explanation of this issue with some supporting citations. This can be found by clicking here.
It TurboTax, you may need to enter this on a screen in the medical expense section labeled Other Medical Expenses. This page should provide entry fields for a description and an amount. In the description, I would write “cost of special prescribed food”.
