Cream together the butter, sugar, and potato starch. Add the ginger to the water and bring to the boil. Beat the water into the butter mixture. If the starch has not thickened, you may need to heat the mixture gently in a sauce pan (or in the microwave.) Stir in the molasses, then let the mixture cool until it is only warm. Sprinkle the baking soda, salt and spice over the batter then beat for one minute. Stir in the flours. The dough should be very heavy, but still able to hold together when kneeded. Chill the dough in the refrigerator for at least two hours before rolling it out to 1/8 inch thick. Cut the cookies and bake for 11 min at 325 deg.

For tips on how to shape GF cookies with cookie cutters see

Making GF Cutter Cookies