In a medium bowl whisk together the GF all-purpose flour, baking powder, salt, and sugar. Add the melted butter and the milk, and mix until incorporated.
Let rest covered in the refrigerator for at least 20 minutes.
In a large bowl toss the sliced plums, berries, brown sugar, and cornstarch. Transfer the mixture to a baking dish and let rest.
Preheat oven to 375°F (190°C).
Droop scoops of dough on top of the plum mixture. Divide as evenly as possible, so that every portion has a scoop of dough.
Brush the dough with cream and sprinkle with 1 tablespoon of coarse sugar.
Bake for 50 minutes or until the crust is golden brown and the filling is bubbling.
Serve warm with a scoop of ice cream or whipped cream.