By root veggies, if you're meaning potatoes, carrots, or onions-- the results will be interesting. I've grown garlic aquaponically before, but I didn't care for the puny-ness I got for the root itself at harvest. However, that plant was part of my first ever set up, and the poor thing did suffer from my inexperience.
I think the better choice for such plants would be a wicking bed sort of set-up rather than a gravel bed.