In theory, you can bake anything on a covered grill, but it's up to you how to regulate the temp- depends on the grill, the charcoal etc. Raw galettes are pretty delicate to be sliding off a peel- you could have a real mess on your hands. The stone might help disperse the heat, but it could get too hot, and likely would leave a black spot (from burned in butter) on the stone- a perforated pizza pan or the like might be a better choice.