I've used my Gri Gri on a 9.8mm Petzl, a 9.7mm Blue Water, and now a 9.6mm Edelweiss and I've had no problems.
I feel I have to add the disclaimer if I'm going to offer input. So...
A Gri Gri should not be used as a no-hands belay device regardless of rope diameter. The friction generated by a smaller diameter rope is less than that generated by a 10-11mm rope. If you are going to use a Gri Gri with a smaller rope, please be extra cautious.