I have heard it said that you can do this, but I've also heard that you only get to deduct the difference in cost between a GF item and the ordinary product. For example, if a loaf of bread costs $3.00 and a loaf of GF bread is $5.50 you can only deduct the $2.50 difference.
In addition, you need to have $2,500.00 in un-reimbursed medical expenses in a year before you can itemize them, if I recall correctly.
Lastly, the differences in cost between regular food products and their GF versions might be difficult to establish to the satisfaction of the IRS if an audit were to occur.
With all of these limitations, I think few Americans with celiac disease pursue this option.