Preheat a grill to medium-high. Grease a medium cast-iron skillet. In a medium bowl, whisk together 1 cup flour, 1/4 cup sugar and the baking powder. Mix in the buttermilk and butter until just combined.
In another medium bowl, toss together the nectarines, berries, remaining 1 tablespoon flour and 2 tablespoons sugar.
Spoon the fruit into the prepared skillet and dollop the dough over the fruit in 8 spoonfuls; sprinkle with sugar. Cover with foil and grill until the juices are bubbling and the top is golden, about 20 minutes. Let cool for 5 minutes before serving with a scoop of the ice cream.