Reviving this post once again...
I seem to get diaherria when I eat more than 700mg of Na a day (a lot compared to other posts, but less than a third of the FDA's recommended intake.)
I don't mean to degrade Fuddruckers integrity but they guessed once for me and guessed wrong, so at least you didn't think it was safe to eat there. I wound up eatting their fries and had a reaction 2 hours latter. So my guess is the are not gluten free.