You can enter these expenses in TurboTax Online here:
- Navigate to Federal > Deductions & Credits > Medical Expenses > Start or Edit
- Enter all the expenses you have as you go through the categories
- On the screen "Tell us about any other medical expenses" you can enter the costs of the special food in excess of the amount of its equivalent gluten containing counterpart. For example if a loaf of gluten-free bread is $6 and regular is $4, you can only deduct $2.
Note that in order to claim medical expenses you must itemize deductions and only the expenses over 7.5% of your AGI are deductible.