Heat the oven to 425°F. On a lightly floured surface, roll the dough to a 13-inch square.
Trim off the corners to make a rough circle -- it doesn't need to be perfect.
Slide the dough circle onto a parchment-lined or nonstick baking sheet. Prick the dough all over. Arrange the apples in neat concentric circles, just barely overlapping the slices, leaving about a 1-inch border of pastry. Dot the apples with the butter and sprinkle with the sugar.
Fold the pastry edges up over the apples to make a 1-inch border. Bake until the dough is medium brown at the edges and underneath and the edges of the apples just begin to brown, 25 to 30 minutes. Slide onto a cooling rack.
As the galette cools, put the jam, water, ginger, and salt in a small saucepan over low heat (or in a glass bowl in the microwave) and heat gently until it just starts to bubble. Stir to blend and remove from the heat. Strain the jam glaze to remove the ginger pieces, or just push the ginger aside. With a pastry brush or a small spoon, lightly glaze the apples and the pastry edge. Serve the galette hot or warm.