Place a nonstick skillet that has been coated with cooking spray over medium heat. Place one 8-inch flour tortilla in the skillet, and top with 1/4 cup of the Gouda and 1/4 cup of the spinach. Put another tortilla on top and cook until the cheese starts to melt, about 2 minutes. Turn the tortilla over to toast the other side, another minute or so. Repeat this procedure with the remaining ingredients.